Are you drinking enough water? Dehydration may be the cause of your constipation.
I read your question, but really only glimpsed over the other responses. So if i’m saying something that’s already been mentioned, then that just brings more relevance to how important it is. So consider this another affirmation.
Drink 1 gallon of water a day, when I did QD over the summer I was drinking about 3 gallons (at the time I had a girl who worked at Stater Bros so I got groceries for relatively free).
Get yourself some Grow!, and use that to add another 2-3 ‘meals’.
What I did to make this easier was get empty water bottles, and carry 2-3 filled with pre-mixed Grow!. When I was ready just re-shake and drink.
Oh yea, a big secret to bulking…get a girlfriend who works at a grocery store. 
You losing weight not neccessarily be a bad thing. Now that you’re eating clean (or so it seems) you probably lost some adispose tissue. That’s a plus.
Another thing, you can get an assload of raw peanuts from Trader Joes or some where similar for pretty cheap. Also you can get apples cheap.
Grow! in water bottles, peanuts, and apples. Three things that are extremely easy to carry, will add calories and easy on your digestive tract.
You have to remember that in a bulking phase the point isn’t to eat a WHOLE LOT (at once)…it’s just to eat all the damn time (thus eating a whole lot).
You don’t want to ADD calories, you want to multiply them.
Instead of 50+50=100
You want 22222*2, etc.
Hope you get my point.
Rome wasn’t built in a day. Or Three meals.
-Xen