Guinea pig help please!?

I have a guinea pig that i bought in september 2009. she looks nice and healthy. i bought another one in the spring of 2010. they are both normal guineas. i think about a year ago, i found something in my guineas hair and i picked it out, i didn't think. it was a scab and i thought OMG! what is it?!?! i didn't think it was a huge thing because she seemed fine afterward. about 6 months ago i found something in my other guinea's fur and i did the same thing, i kept meaning to look it up but i always forgot about it. finally i decided to and i found out it might be mites. i'm taking them to the pet store where i bought them by next week(thats the only time i can go).( they know a lot) and then if anything is wrong i'll take them to the vet. when ever i touch a certain spot on their backs they flip and start biting what is in front of them. i'm really scared (because what i found on the internet) and do you think that it will be okay if i go to the vet, will it be too late please help. and yeah i know i shouldn't have picked it out but it was a moment of desperation. thank you!