Suggest Treatment For Abdominal Pain

Hi! I'm 25 years old and ever since I had my period when I was 12 I never once had a regular period. The number of days of my period are always diffrent than the month before. We are trying to concieve but its already 3 years and nothing happens. I also tried to have a check up a lot of times and they said that I dont have any abnormality but doctors here in Japan doesn't like when you're trying to tell them they might be wrong. I'm not stressed nor overweight so I'm wondering what is wrong cause my partner has a child with previous marriage so its unlikely that there's something wrong with him. And just last week on my 38th day I had my perod and there's a stinging pain in the left part of my belly. It was dark brown near black. I dont think it's dysmenorrhea cause whenever I'm having my period all parts of my belly aches but this time it was just a small part on the left side, somewhat exactly sized with an end of a finger. Please, what do you think might be wrong with me?