I've been experiencing sinus post nasal drip and a feeling of fullness/gas while taking setraline/zoloft. Are these side effects of the ssri?

Yes, they can be. Zoloft (sertraline) can cause many types of gastrointestinal symptoms like nausea, vomiting, diarrhea, bloating, etc. Less people realize it can also cause rhinitis -- but it can. Some of this may dissipate with time.