Weight gain in 3rd trimester

Sorry, just have to rant, I've gained 19 pounds so far and I'm 33 weeks. I feel like my knees can't handle any more! I wasn't at the weight that I necessarily wanted to be at when I got pregnant, and I've done well with not gaining a ton of weight, but walking up and down stairs has been killing my knees. And sitting down on the toilet? Sooo painful! :(. I can't imagine gaining even more between now and 40 weeks!