What Causes Pain In Abductor Pollicis Brevis Muscle?

Hi. I have been having pain in my hands for the past 2 weeks or so. It is in the abductor pollicis brevis muscle. Mostly hurts in mornings and night. It helps in the morning if I lay them flat on the matress, put my pillow on top, and rest my head over them. At night I have to massage them often. I do take artane, Seroquel and lithium for my bipolar.