Period Masturbation pain

So whenever it's that time of the month I feel more horny which I've hear is very common and normal. I've also heard of ppl masturabting and even having sex during their period, but whenever I masturabte during this time, I have so much pain kind of around my bladder. Does anyone else experience this? Tonight it was so bad, the worst it's ever been and I was terrified, as if I had to go to the hospital or something. I put a hot rag on the area tho and after awhile it subsided. Please help if you have this too. Why is this happening??