I'm really itchy under my breasts and on the top of my thighs. can i have yeast infection under my breasts but can that spread to only my thighs?

Yes and yes.: Ringworm or candida infections can be spread from one place to another, if those places stay warm/ moist/ airtight. Dry skin or bacterial suprainfections can do the same thing. I would consider using miconazole or monistat cream in the folds under your breasts until the itching is gone. Then use a desiccant cream like Desitin (zinc oxide diaper cream) to keep the area dry so the yeast won't grow back.