Is he chewing on anything he possibly can? I know at 11 weeks he probably still has problems getting things to his own mouth, but if he's trying like crazy, then it may be teething. If it is teething, if you offer your finger up for chewing on, they'll usually go for it quite happily, letting you feel his gums for any lumps or sharp spots.
It can be difficult to tell when a tiny infant is having diarrhea, as normal baby poo is so watery and runny anyway. But, the fact that it's giving him a rash is a great indicator. My kids have had diaper rash bad enough that they needed tylenol to sleep because it hurt so bad. Find a diaper rash cream that has zinc oxcide in it... that means it'll be white. The thicker the stuff the better. Get his bum really clean by soaking in water with some baking soda. Rubbing will only make it hurt more. Then once it's good and clean, slather the cream on really thick. When you change his diaper, wipe off the ick but leave a layer of the cream on, then add more to it before putting the new diaper on. It'll help clear up the rash and make him more comfortable.
I'd also recommend going back in on Monday when you can see your regular doctor that you trust. If you need to see someone sooner, I recommend the After Hours Pediatrics clinic on 143rd and Metcalf. They have Saturday and Sunday hours, as well as evening hours. It's walk in only, but they have two good pediatricians on staff and they're great with kids. They may know more than the doc you saw last night.