painful period

Okay so basically, my period is very weird. I'm 17 now and I started my period when I was 14. However, it's always been irregular. I know that after a few years, your period should regulate but mine doesn't. Sometimes my period doesn't come for 4 months. It NEVER comes at the same time every month so I never know when it's coming. But, when I do get my period it is the most awful thing ever! It lasts for 7 days. I have a heavy flow each day. The last 2 days is when it gets lighter. And the cramps I have are excruciating.. I have the miss school and stay in bed because I simply can't stand up straight. I get lower back pain and the pain also reaches my thighs and hips. I've been the my doctor and he recommended that I take birth control pills which I have been. I have been taking them for 5 months already and they do not work. I still have awful cramps,heavy flow, and my cycle is still very irregular. Also I started to feel naseous everytime I'm close to get my period. My doctor doesn't know what it is so I'm asking for anyone to help me because I am honestly so tired of dealing with this... All I want is for a regular period with 3 days of heavy flow.. (sorry if this was super long lol)