Open Question: Should I be worried, Pregnancy? Cyst? I don't know whats wrong.?

I've pretty much been having unprotected sex since the start of the year, and a bit before. I don't use any birth control, and the only method he uses is 'pulling out' Which has been successful at times, and others weren't. My periods are VERY irregular and can miss 2 months at a time, which makes it very hard to know if i have to worry or not. I've missed my monthly over a week now. and only recently i have been experiencing problems such as fatigue, lower back pain. Spotting (but im not sure) about two weeks ago, i think. I've been feeling nauseous and had the worst headaches, and mood swings are out of my control. And lately intercourse has been painful Im very far from worrying though? There is a few things that could be wrong. Cyst, pregnancy, STI etc And i need answers, if you don't mind saying your opinion.