Anyone know what bit her?

OK, so for the last two mornings, Juliana has woken up with big welts on her legs. This hasn't happened before and they are bothering her so much!! I was wondering if anyone had any idea what kind of bug may have bitten her to create these welts? And how concerned should I be? I know that my mom had gotten a couple while she was visiting, but they went away without any problems. I just want to make sure!!! Any feedback would be great. Thank you.