Welcome. A couple things. Your time line doesn't make since. If you got your snake 20 days ago and you fed it on day one then then its been almost 3 weeks. Each BP is different and they are going to defecate when they defecate. Unless you start seeing a large bulge at the vent you should be fine. You should feed on your schedule and not worry about bowel movements so much. My BP will often save up a few mice and leave me a great big surprise when she sheds. I would not start worrying for at least another week.
Next is temps. Can you show us a picture of your setup. You should have two identical hides that are tight fitting, one on the warm side one on the cool side. The warm side should be a bit warmer. I shoot for 89 to 91 although lately I am thinking about dropping that to 88 to 90 based on my snakes use of her hides. Your cool side doesn't seem to bad and if you bring up your temps on the warm side just a little you should be perfect.
Humidity, if we see a picture of your setup we should be able to help. If you are using any lamps that give off heat you could stop as they suck out humidity. I use a florescent fish tank light for my lighting. BP's are nocturnal so the lighting is all for you not for your snake. Also you could try reducing the ventilation. If you have your snake in a tank you could easily cover 75% of the top and have great ventilation.