I am 15 years old and was just put on the birth contol shot about 2 and a half months ago. I didn't have any periods after the shot until 5 weeks ago and I've had my period ever since. My period hasn't been very heavy at all but when I do get a lot of blood is whenever I'm sexually arosed. I haven't done anything remotley sexual since I started my period 5 weeks ago and yet when ever my boyfriend and I are kissing pationetly, I find afterwards that my underwear has quite a bit of brown blood on it. I have been tested for STD's and have none so I'm confused and worried that this might be something serious. If anyone has any input on why my body is acting so strangely it would be greatly appreciated!