For over a year now when i lay flat on my back i feel a lot of pressure and some pain in my pelvic bone. should this a serious concern?

Maybe: This appears to be muscular tension type distress developing when you lie perfectly flat, and if the discomfort is immediately relieved by changing position thereby taking the tension out of the muscles, that would be reassuring. My feeling is that there is no serious problem, but if you have ongoing concerns have a gynecologist perform a pelvic examination.

Supine pelvic pain: Pain x 1 year should be evaluated . May be sciatica, (see if a pillow under your knees helps), hip or ileitis/ celiac (GI origin though one would expect GI symptoms) , or ovarian ( have seen vaginitis with mycoplasma cause pain) /renal. Ffollow up with doc for good exam to localize consider CT abd/pelvis and or MRI LS spine, hip films and ESR,CBC ,U/A possible food intolerances testing