A.,
I know quite a few people who "swell up" from mosquito bites. I don't know if it's an allergic reaction, more of the mosquito venom or what. As long as it doesn't cause a fever, incapacitate her or bother her too much I wouldn't really worry about it.
To make HER more comfortable, I would only apply a cold compress as needed, (my kids liked "Mr Happy" we got at Cost Plus World Market), or a topical treatment like Calamine lotion or Dermoplast spray (this was given to me at the hospital after having my first child - it can go ANYWHERE and is nice because you don't have to rub it on and aggravate bites/cuts/irritated skin - even private parts).
Personally, I don't give my kids ANYTHING orally unless my ped says it's ok. Many drugs have some side effects that aren't worth what you are trying to treat - including creating a dependency, dehydration, etc. I don't know about Bendryl, just giving you some examples. So, please be careful with such a young child.
As for prevention, I have heard that fabric softener dryer sheets repel mosquitoes. Just "wear" one in your pocket. Also, if you want something stronger, we buy an oil from Erewhon IT DOES NOT GO ON SKIN. We put it on the tips of shoes, brims of hats, etc. Just a few drops repels almost ALL mosquitoes at festivals (I swear). Again, nothing on their skin, therefore, no chemicals getting absorbed. River raft guides recommended this to us on an outing where the rest of us wearing "Off" were getting eaten alive. This oil is amazing and everyone in our family puts it on and forgets about the bugs.
Good luck.