Whats the stringy, sticky, mucus like substance that is developing under my eye lids?

Hmm, I was going to say conjunctivitis, but if it only happens when you get hayfever maybe it's something to do with your allergy. You can get conjunctivitis in one eye or both, and you usually get a thick sticky discharge like you're describing. But if it's really bothering you, go see a doctor or even a pharmacist, they will be able to help further.