why do i get nervous for no reason?

I started Zoloft a cuple of months ago and recently uped the dose from 50 to 100 and since then iv felt no different apart from that I get a really bad feeling I think its nervousness, it feels like when you are waiting in the teachers office knowing you have just been found out that you have done something wrong and know you guna get in trouble or you know you have to do something that you really don't wana do, it hurts my stomach too its an intense feeling :/