What would cause a thin endometrial lining? i have had multiple ultrasounds and many points of my cycle and my lining is always thin only 4mm sometimes 5. not on bc pills. havent been for 3 years. struggling with infertility. could this cause it??

Therein lies a strong "clue". The endometrium grows or "proliferates" during the first half of the menstrual cycle and becomes more lush and "secretory" with glands during the second half of the cycle after ovulation when progesterone is secreted rather than just estrogen as in the 1st half of the cycle. If the biopsies were done in the 2nd half of your cycle, then ovulation may not have occurred

If your periods are regular (24-32 days apart), and the values from ultrasound are "always" at about "4 mm", it suggests that ovulation is not taking place. If you do not have regular cycles, and only have long gaps between bleeding episodes, obtain a referral to a reproductive endocrinologist for evaluation and treatment.