My mother has a big black purple ish spot on her leg. i don't think its a bruise. i remember my friend said to ask a docotor cause it might be a clot?

Go see her doctor: Take your mom to see her family doctor who can make the diagnosis. There's not enough information w/o seeing spot so all i can do is speculate which does you/her no good. Be sure to bring in all medications (prescription ; over-the-counter) as well as dietary supplements eg herbs, pills, powders, etc, as spot may be a drug-drug-supplement interaction.