Suddenly a small marble sized cyst appeared on my groin between my thigh and public area. it is moveable, hard. not painful but irritates due to the area. i was playing tennis in the heat and had tights on. could it have been caused by sweating ?

That's an area of lymph nodes, nerve and blood vessels. It could best be evaluated by a vascular surgeon whose office likely would be equipped to image the area with an ultrasound machine.

It sounds like an inguinal lymph node on location. Irritation might have contributed to some extent. Best to start with would be your primary care doc, who would be able to examine and coordinate locally further tests and/or referrals as needed.