“Question about antidepressants working quickly?”

I'm a 26 year old female and my weight fluctuates between 115-118 pounds. I was put on antidepressants for anxiety and depression. I also have PMDD, so my mood gets crazy around my period. I have pure OCD too, I mentioned that to my nurse practitioner but it wasn't put in my medical file. I was on Zoloft for 4 days, at 12.5 mg, but could hardly stay awake and slurred my words. I got changed to venflaxine 37.5 mg extended-release capsules. I've only been on it for 2 days. I've noticed that orgasming is very hard now, as it feels numb, is it possible that the few days of Zoloft and now venflaxine has already effected me? Some places say it takes weeks for antidepressants to work, but some places say it depends and can work in days. Can people be more sensitive to antidepressants than others? And is there an antidepressant that doesn't lower libido? I asked about welbutrin, but my nurse practitioner said it may make my anxiety worse. But people who reviewed it say it sometimes helps with anxiety, it that true?