What could be causing pain & stiffness in my big toe joint?

Technically : The big toe joint would be the interphalangeal joint not the mpj, but, I'm not sure which you are referring to. If the ip joint, I'd look to see if the toe was straight or not. If not internal compressive forces may be irritating the joint. If mpj is bothering you look for dorsal spurring to be a cause, I.e hallux limitus. Gout also affects this joint.