What was your diet like before you started eating less carbs? And what is it like now? This will gives us an indication of whether you could be suffering because you are eating less carbs.
You had some weetabix, two bananas and some chocolate before and during your 100km spin so that's not low carb anyway.
i was a carbaholic ha ha cereal twice a day maybe 8-10 slices of bread lots of chocolate chips potatoes burgers you name it lots if not all processed foo.really really bad but a hell of a lot of carbs.
iv cut out most carbs except before a spin
Ok that's a serious change 
It's probably just taking your body a while to adjust to the change in diet I reckon. If you were participating in any other sport you'd probably be feeling the same way.
I wouldn't worry too much about it (easier said than done obviously if you are having massive aches and pains near the end of the cycle!) There is still another while to go before the ROK so your body should be in much better condition if you keep the training up, and stick to having less cereal, bread and processed food.
You call eating pasta and weetabix low carb? 
Did it for a summer two years back. Made a big difference on the bike and with weight loss.
But off the bike I was tired pretty much all of the time. Recovery took longer.
By September I had shifted about a stone but stopped. Was simply wrecked all of the time.
On the one hand I felt healthier and was lighter. Enjoyed my food.
I hated the creeping feeling of sluggishness as time went by.
I sipped and went back to sugary carbs. Don't feel tired but am as fa as fook so cycling has deteriorated.
But the idea of a low carb diet is to move from burning carbs to burning fat and I woudl have thought his carb consumption is still too high
Otherwise why do it....just watch your sugar & fat and calories if you want to lose weight...
The switch from burning carbs to burning fat for fuel can be very difficult for some
I'm guessing you need to ad BCAAs to your diet. a scoop or two in your water while training will be plenty. I've read recently that amino acids while training can be more effective than protein
But then again ....I think it all depends on the individual because have read of this
...its not the amount of carbs that is the issue... its the quality
http://www.dailymail.co.uk/health/article-3100985/Could-eating-carbs-key-longer-life-Diet-low-protein-produced-benefits-cutting-calories-40.html
I'd focus on low GI carbs rather than cutting them out entirely.
Carbs are necessary.
Proteins if you eat enough of them turn to carbs...so it is impossible to cut carbs entirely but a low carb diet is touted as a very good way to burn fat however it is hard to maintain
It has worked for me
But I acknowledge it does not suit everyone and would suggest a switch to high quality carbs like brown rice, sweet potatoes, porridge, quinoa, whole grains breads , legumes & lots of fruit & veg
GI is effected by how you cook the food and what you pair it with ...for instance the GI of a baked potato is significantly different (much higher) to a boiled one
And the GI load of you meal can be more significant ...ie combine good carbs with protein to lower GI load...So eating pasta by itself is not a wise as eating pasta with a meat sauce or a protein /veg sauce sauce
http://www.eatingwell.com/nutrition_health/weight_loss_diet_plans/diet_exercise_tips/how_to_eat_lower_on_the_glycemic_index
Oh I agree that the OP's carb intake is still too high but reducing the amount of junk that he said he was eating should help him lose weight.
ok guys i started a low carb diet one week ago.its only to last 2-3 weeks just to shift 6-7 pounds before the rok.i aint over weight just like to be pretty lean doing it.thing is this week i been doin a couple 40-40k cycles with some pasta couple hours beforw and have seemed grand.yesterday i got up for an early morning cycle had some weetabix and yougurt before my cycle(to early for pasta plus heard you need to eat it couple hours before cycle) through my cycle i had 2 bananas and a bita chocolate.i done 100k but feck me the last 15-20 my legs were in pure agony.iv done 200k before and didnt feel that bad.is it the lack of carbs that would have done it.maybe big bowl of pasta the night before would have sorted me?