I expect that it is the other rat that has made this change in behavior. If i introduced another rat with Tia, she would go mental. If it was her teeth, you'd probably notice her gnawing the bars of the cage or a wood block if she has one, and you'd probably notice her have difficulty in chewing her food and she would lose weight. Often, introducing small mammals that are not from the same litter is a risky thing to do as, especially with rats, they are prone to squabble with each other. I once had two roborovski hamsters that were from the same litter, and we had to separate them as one attacked the other quite badly. If you ever notice them standing and staring at each other or squealing then separate them immediately. Sometimes, rats do playfight, but believe me, you will know when it's the real thing. I would suggest separating them now just in case it does come to that, you may find that you'll have to 're-train' your rat into being handled again, but that can be done fairly easily using treats and gentle hands.