Weird pimple/blister (Possibly mole)

Hello, about a month ago i noticed a weird thing on my back. It looks like a mole, but acts like a pimple filled with blood. It gets bigger, then flattens down, and later gets bigger again. Today i accidentally scratched it and blood came out of it, and it flattened itself again. This tells me that its more like a blister that fills itself with blood. I had a similar thing a year or even two years ago and it disappeared on its own. But this time im worried it might be something serious since i first noticed this one a month ago and its still coming back. It looks like a scab when it flattens, but when it fills with blood it looks like a weird mole. What could this be?