Can increased exercise cause a stall?

I am just under 3 months out and have been stalled for the last 11 days (playing with the same pound in this time). I have lost 43 lbs. The last couple weeks I have majorly ramped up my exercise in intensity (500-600 cals burned) and frequency (6-7 days a week), which really makes this stall even more frustrating. Is it possible that my body is stalling out because of this increased exercise and still relatively low caloric intake? I generally eat 800-900 cals. I am hitting water and protein goals 90% of the time. 

Yes.  Lots of possible factors...

Increased water retention as your body adds muscle, increased weight of muscles vs. fat, a normal stall where your body is shifting fat around at the moment, and there are more possibilities.  A normal stall around three months, without even factoring in exercise, is very typically.

The bottom line is you're likely not stalled at all.  You are probably lossing body fat but gaining something like muscles or water as an offset.  This won't last forever and will evenutally pass so don't sweat it.

We all get very focused on the weight number while in the loss phase but all that really matters is body fat %.  When you're exercising you're doing more than just burning off fat and that shifts the formula around a bit.

I am going to hang on to that train of thought - that I am losing fat and gaining muscle. A friend hadn't seen me in a couple weeks and asked how much I had lost since I had last seen her because she thought I had lost a lot. It was probably only 2 lbs that I had lost in reality which wouldn't be noticeable. So really, I must be losing fat. 

You may also simply be switching to a stair step loss pattern as well.  I lost consistently at first... but then I would go up to 21 days with nothing, then rapid loss over just a couple days.  My monthly averages always held up though.  It can be frustrating at first, but if you use mfp track your weight and look for patterns.  It will help you get through it.  If you are working you plan, it will work :)  Last thought, 3 months is a very, very common stall point for many. Your body might just be catching up.  Measure yourself... you are likely losing inches. 

I was dumb and haven't taken measurements t:( Really should do that so I can have the gratification of inches lost during these stalls!!

Whenever you increase the amount or intensity of your exercise your muscles will react by hanging onto excess water - lots of it!  You are not eating too many calories or too few calories and your body is NOT going into a stall because it thinks it's starving (total BS, don't fall for that story).  

You may want to increase some of the good fat in your diet (which might raise your calories a bit) by adding some nuts, seeds, avocado, butter or olive oil to your diet.  I always found this helpful in moving me off a bit of a plateau.  

Hold the line, continue to do what you're doing.  Your body will adjust within a couple weeks and the scale will move.  I promise you that you are losing fat, it's just not showing up on the scale yet.  Taking measurements will help you see this too.  

Thank you... My head knows this stuff but it's still hard to wrap your head around when you are doing what you should be and aren't getting the instant results. I will just keep on trucking :)