One of the side effects of Zoloft can be insomnia, but it seems rather odd that it would begin affecting you after 2 months.
Everybody goes through bouts of insomnia, so it's possible that this has nothing to do with the Zoloft. I would advise giving it another week. If it has not resolved itself by then, speak with the doctor who prescribed the Zoloft. Even though you are on the lowest dose, he/she may have you cut the dose in half to see if that helps.
Insomnia is a real drag, but try not to let anxiety become a part of this temporary problem.
What time of day do you take It? You might also discuss with your psychiatrist splitting the dose in two, taking half in the morning and half in the evening to see if that evens you out.