E.K.
Hi N.,
My daughter did the same thing. When I asked the doctor, she just said her cycle is changing and she may just want to see you. She'll get back to her own schedule again. It shouldn't be a concern for apparently all babies do this at some point.
In regards to the mattress, I would get a mattress cover (it's comfortable and it gives just the right cushion) and keep it there until she graduates to an older bed.
Good luck!