Head Shaking, runny eyes

Since I adopted my cat a few years ago she always seems to be shaking her head and getting runny eyes off and on. I have taken her to two different vet's, and had bloodwork done on her to make sure she is ok. 

The last vet said her ears look great, she has no mites or fleas (Indoor cat) and gave me an antihistamine for when she flairs up. I'm still a worried mom and wonder why she does the so often. Any advice? 

Just bumping this up since you've not had any responses.

Does the antihistamine work when she starts in?  If so, then I'm guessing perhaps it IS allergies.  Is it happening NOW?  I know my allergies are at their worst this time of year. 

BTW, what antihistamine did they prescribe?