^^^^ great answer by rowan......the internet has got a lot to answer for in the field of medicine. To pit sarcomas in perspective, a GP will be unlikely to see on in his professional lifetime and you will have more chance of winning the lottery than getting a sarcoma of any kind.
Now...back to your question:
Not enough information.
Where is the lump? Is it attached to skin or muscle? Does it pulsate?
I like tambo's suggestion but it seems to be in the wrong place for a bursa.
How is it affecting you...pain, can't walk.....??
What happens next?
1) If it is affecting your life.....then nothing....monitor the situation.
2) Aspirate it......painless removal of the fluid.
3) Excise it (cut it out) depending upon where it is.
There you go!