Swollen hot chicken legs? What do you think would cause them?

I have a chicken with Swollen, Hot legs and is now starting to walk pigeon toed. Poor baby is actually starting to step on her own feet as she walks. She's 3 1/2 months old & not laying yet.
She eats well. Plenty of Fruit, grain and bugs. Bugs however are becoming few and far in between as she sits more now and is only getting the bugs that seem to cross her path. She tries very hard to keep up with the small flock. Moves with them and sits. The coop is very clean and they wonder about the yard throughout the day. A friend looked at her and said she is at a very healthy weight, she looks good, no mites and flock doesn't have warms. My friend is very puzzled by what might be wrong with her. PLEASE, does anyone has any ideas?? Do you think Antibiotics will help? I have Penicillin 250 mg on hand but wouldn't know the dosage for a chicken. Any Ideas??