Why am I bleeding after sex?!

Why is this happening to me? After sex I’ll start spotting and then the next day I start my period or at least I think that’s what it is. I’ll be spotting for several days and then it goes away and I don’t understand why. It started after my 3rd baby, he is 7 months now and it’s still happening. The Dr said it may be the birth control pills I’m on but it’s been going on since before I started them and before the pills I got on the depo but I only did that one time. It’s just crazy I haven’t been the same since this 3rd baby. I could go on about all the pains I have. Could anyone here have any insight as to why ?