Exhaust leak?

Has anyone had experience dealing with an exhaust leak? Whenever my truck went to do parked regens or those little mini regens the last few days I could smell exhaust in my cab. It actually woke me up the other morning in Wisconsin and I had to roll the passenger window down all the way and stick my head out into the cold air. At the time I thought it was merely sucking in air from the truck next to me as it has done from time to time. Well this morning the truck next to me wasn't running and there was no one on the other side of me so I ended up having to shut the truck off mid regen. I figured it would be ok as it can do a passive regen when I start down the highway. For some reason it refused to regen after nearly two hours on the highway. So I pulled into a truck stop where there wouldn't be any access to another trucks exhaust and began a regen. Sure enough ten minutes into it I begin to smell exhaust in the cab so I immediately shut the truck off and call breakdown. The mechanic they send out does a regen but says he didn't smell any exhaust so now I have no idea what to do or what's going on. All I know is I've had a splitting headache since this morning that has not gone away and I feel nauseous and sick and would just like some outside opinions.

 

Yeah this truck has the pipe end under the back of the cab too but I've had it for four months now and this has never been an issue. I tried looking at the exhaust system but can't see much myself. Talked to a mechanic and he said maybe the regen system is clogged and pushing it back into the truck when it does regens. Ok, but I started her up this morning and just about fifteen minutes ago I could smell exhaust again (it wasn't doing a regen) and had to shut her off and open the windows. If it was coming from another truck I should still be able to smell it when I open the windows but I can't so it's got to be from my own truck. Well my headache is back again and I don't know what to do about this.

I did notice yellow liquid pooled under the end of the exhaust pipe; is that normal? Or a side effect of regens?