I am 29, i do have pains on the right side of my stomach about one to two weeks before my period, i went for an ultrasound the doctor said the lining of my womb is thick. what could this mean?

This can happen normally in the menstrual cycle before it sheds as menstruation.

The timing of your pain is suggestive of ovulation associated pain (mittleschmerz) which is relatively common and normal. It sounds like the doctor was simply observing a normal appearance typical of later in your cycle. If he meant that the lining was abnormally thickened then growths like polyps or fibroids are a possible explanation.