Help me out please?

I haven't had a period since April. 
Sometimes my periods don't come when they should so I'm not sure when I'm next on! I've had to change glow twice because I haven't had my period when it's said to have one lol. 
But I've been having cramp pains, my boobies are a little tender. My back aches like hell. But my pregnancy tests still give me a BFN!! took ovulation tests and they come back postive still, I took 2 last week an came back postive. Not sure what's going on? If someone could help me lol