It isn't uncommon for your periods to take time to get back to regularity after pregnancy, even though it returned so quickly (which usually happens when you don't BF).
Give your stress levels lately too, again, not uncommon for it to mess with your hormones, and the Zoloft will not be helping.
The side effects of the Zoloft itself also can be what you are reading as pos pg signs.

You REALLY need to sort out with DF the use of contraception. You are not ready for another baby, you know this, despite how much you want one. And if he has expressed a desire NOT to have another one right now, then you need to start taking responsibility.
Condoms are easy, if he won't wear one, then no sex. Simple.
As you aren't BF you could go on the pill, you could also use a diaphragm.
There are so many options, burying your head in the sand and hoping you will get pregnant despite knowing DF isn't ready and neither are you, is not going to do any of you any favours.

Time to start acting like an adult and take responsibility.