The rash could be due to sweat since its been warming if you have been. I would ask the doctor about it they could prescribe you something for it. As for the constipation I'm 16wks 3 days and I've had it ever since I started prenatal vitamins at 4 wks I drink a glass of Apple juice either everyday or every other day and it helps out a lot, or try prune juice like Lexxus suggested.
Fiber powder helped me I wouldn't poop for days until I started putting fiber powder in my juice now I poop everyday so now I only need one spoonful of powder a day and I'm fine.
Im the same exact way im also 14 weeks and a day I drink prune juice everday or every other day to help me use the bathroom so try that :)