What Causes Hard Bruise On My Inner Thighs?

My inner side of my knees have been hurting for a few weeks, as if bruised but didn't appear it. In the last week iv been gym once, and played netball last night. The pains faded in the week but started hurting again when playing last night. I woke up this morning to big purple bruises on my inner knees, hard like egg shaped area behind it. What is this? My left knee being worse than my right.