Hello! My Previous Period Dealed And Came On The 33rd

Hello! My previous period dealed and came on the 33rd day, and am usually on my period on day 28. Strangely l had severe left lower abdorminal pain...a burning like pain which worsened whenever my blooder was full. When the period finally appeared, l only had one full day of bleeding, with spotting on the first and bleeding on the second day. I took a pregnancy test, both the urinary and blood tests. I am 35years old. I went for a scan and a mass of fluid, 1.8 cm was found in the left adenexa reign. On day 3 of the next cycle l had another scan and very mimimal fluid was found in the pouch of Douglas. The doctor confirmed l had no sti's but am concerned about the left lower abdorminal pain which comes at least a weak prior to my period and disappears after my period. Could it be that my period is not cleaning totally? Or what could it be?