My thighs seem to be swollen.....

[Grog]

Four months is enough to start building muscle.

I would really doubt that it is lactic acid buildup. Water retention? Maybe just being properly hydrated. I say get new jeans (hard I know for a woman with actual muscles instead of pogo sticks for legs) and keep riding!

[Eden]

I'd say its probably muscle... It's not lactic acid, your body clears that away relatively quickly. If your thighs feel tight just after a ride, and at that time you notice your pants feel a bit more tight around the thigh, but it goes away, then it could be water retention. Otherwise pat yourself on the back you are building muscles!

[Starfish]

Yep, I bet it's muscle. I think it is possible to see new muscle in 4 months.

And, if your legs are inflamed, try sitting in an ice bath after your rides. It sounds horrifying, but ends up being wonderfully soothing. (I wear sweatshirts and stuff on my top half, and make sure the bathroom is toasty warm.