I registered for an account just to talk to you about this. (I've been lurking for awhile!)
My daughter is 20 and we literally JUST got off this same rollercoaster 3 weeks ago! She found a lump in her thigh muscle, not on the surface of the skin, but well within the thigh. I had no clue so started Googling and lo and behold what do I find?! Nothing but bad news. I panicked and was a wreck thinking my poor daughter had the worst thing imaginable because that is literally ALL that comes up when you Google.
I was convinced it must be sarcoma and luckily in ATL we have a sarcoma center at the University Hospital which is excellent, so instead of fooling around with our Primary Care Drs I made an appointment with an oncologist. He immediately sent her for an MRI (with and without contrast, did the leg and the pelvis too). Waited for over a week to hear results...inconclusive...sent her to another oncologist. He ordered a CT guided biopsy. The Radiologist came out and got me the moment they had her under the CT scan - he said it was a Vascular Malformation called an Angioma, but for our peace of mind he would biopsy it anyway. They took a BUNCH of tissue samples and it was ALL BENIGN! The 2 weeks waiting for the final biopsy were the hardest!
The main thing I want you to know is, there ARE definitely other things it can be when you find a lump. I didn't believe it, but it is true. But the oncologist did tell me that we were very smart for getting it checked right away. Angioma's in the thigh are rare (they are normally in the brain/spine) but can be anywhere. Hers is 4cm and has probably been there all her life, but may have recently grown. She does not have to have it removed unless it ever bothers her.
Remember, there are also LOTS of benign lumps and lipomas. Unfortunately with a lump the only way to tell is a biopsy.
Good luck to you my dear! Try not to worry, but I have intense HA too so I understand.