Woke up with yellow blister on my arm, I cant tell if its a bite or a reaction, please help

I woke up this morning and went to my morning class, and after class i felt a bump in one of my sweater sleeves.

I pulled back my sleeve to find a round yellowish blister stuck on my arm, bigger than a pencil eraser and sticking up quite a bit.

However there is no pain, and while there was some redness around it, it seems to have dissapated a little, but the yellow blister is still there.

I dont know if it's a spider or other insect bite, or if its just a reaction or some other odd skin condition.

It's been the same size for the past 12 hours, does anyone know what this could be?

I dont know whether to be afraid to go in my room where the (possible) creepy-crawly is, or if it's a reaction of another kind.

Please help!

Was it just the "blister" or was it red and sore in the surrounding area?

I'd say the chances are good it's a spider or some other insect bite since you only have the one...so it's not like you're breaking out with anything.

Here are some spider bite pics...any look familiar?

http://www.brownreclusespider.org/black-...

http://www.garysdetecting.co.uk/spider.h...

http://brown-recluse-spider.ascendedheal...

http://www.badspiderbites.com/images/bro...

http://www.pathguy.com/lectures/brown_re...

how about this one?

http://www.museums.org.za/bio/images/spi...

Did anyone give you any medicine or ointment to put on it?