Hello, I Have A Lump On A Musle (seems To

Hello, I have a lump on a musle (seems to me the tendon) near my groin aprroxamatly 1inch by 0.5 inch and it bulges out a centemeter when lying flat. when sitting it dissapears but i can still feel it. its as hard as a rock but doesn't hurt, there is also a surface rash stretching from the location to the edge of my hip. I am a fairly skinny man, and i did cross-counntry running last year. I am wondering if this is is an infection, muscle tear or just normal. (although the opposit side has no lump)