I have had a cyst thing on my leg for years, never hurt, never a problem, it just changed sizes. but now, it is red around one side, very tender... is this normal?

Redness & tenderness indicate inflammation and possible infection. I recommend having a physician examine & advise you. In the meantime apply warm wet compresses, pat dry and apply an antibiotic ointment. If increase in pain,redness & swelling, the lesion may need an incision&drainage by a physician or surgeon in the office or in the ER.