My girlfriend had a similar experience, though I don't know about the sex/tampons part because she is single and hasn't had sex in a loooong time, but anyways, she was diagnosed with kidney stones and when she was on her period she felt like she was dying---the only thing that would help her was a hot bath. Apparently the swelling from your period pushes on your kidneys and causes the pain to flare up really badly, and it affected her moods--which makes sense.
I hope you can get it checked out soon. My friend was in the same boat you are, she is the sole breadwinner and has to support her two daughters,so she was sooo scared to get it checked out because of the potential financial problems it could lead to, and she thought that she might have cancer. I called her everyday until she finally went to the doctor for it, It was so worried. I hope you get answers. She even said that she felt so much better knowing what was wrong with her as opposed to constantly feeling stressed and worried about it.
Keep us posted!
Never mess aroound with those kind of symptoms......do they have a free Planned Parenthood or service you can go too? Look online for free stuff for women in your town