Bird droppings or anything like that would probably have color and smell/taste really bad. I think it could have come from a lot of things, but water is everywhere. :) I know how you feel to some extent, my boyfriend gets wigged out by things like that: touching something he thinks is dirty, house hold cleaning supplies, gasoline, etc. I used to worry about things like that too when I was little. You're most likely perfectly safe. I've heard of people eating all sorts of weird things and being fine, so I'm sure something getting on your lip isn't going to hurt you.
When I was little all we ever did was play in the dirt, and I know we got that in our mouths at some point. lol We are all just fine though. Once I ate really old pine nuts that were in the pantry, but I didn't know they were old until after I had chewed them. >.> Bleh, but I was okay.
Another time -- gross I know -- the neighbor's son, who was probably 3 or 4, stepped in dog crap in our back yard and instead of washing his foot or something he wiped it with his hand and then wiped it on his clothes. Ain't nobody going to convince me he didn't touch his mouth later. lol Their son was perfectly fine.
A little while back my boyfriend was doing some kind of yard work for his grandfather and got splashed with gasoline and was really really nervous about it hurting him somehow. We looked it up to double check -- though I was pretty sure there was nothing dangerous about gasoline, and he only got it splashed on his lip -- and everything he found said there was nothing dangerous about it. There was even a story about some guy drinking a bunch of it o.O but when the guy went to his doctor, all the doctor said was that your body can't absorb gasoline, you will just have to wait for it to pass through your system, and that he would probably have an upset stomach till it did.
Just think of all the stuff little kids eat and all the stuff people eat on accident, but they are all okay. If getting something weird on you or getting something funny on your lip was really dangerous you would hear stories constantly about it because it happens so often. :) I think you're okay.
It definitely doesn't sound like it's animal droppings of any kind so it must be water from something. If you think about it, if it's not water and not animal droppings, what could it possibly be that is really dangerous? It's not like there was a guy in the tree trying to drop poison on you, you know? I really don't mean to make fun or anything; please don't think that. All I mean to point out is that there are very few things it could be and none of them are dangerous. :)
I hope you feel better. You're alright. :)