Bleeding after sex

So every now and then when my partner and I have sex I bleed afterwards, however the last time it was really bad and I bled for a few days. I am a lot smaller then my partner and it’s only just stopped hurting when I have sex, so has it got to do with me being so small or is it another reason? I bleed every time after we do doggy (we’ve only done it a few times), will it be like that forever or just until I do it for a while? Thanks girls xx (p.s. 100% not my period or anything like that)