After period bleeding

Yesterday I felt some kind of cramp, very brief, and then some blood on my underwear. I just finished my period 3 days ago.

I usually never have very painfull cramps during my period but this month I spent one whole day in bed feeling very very bad. I take the pills mostly as a treatment for polyquistic ovaries, I've been taking them for almost 4 years now.

It's been almost 4 months since the last time I had sex.

I don't know what's going on, but I'm really scared. Does anyone know what it could be?