It's extremely common to struggle to gain weight with Crohn's. I'd add in some extra nutritional drinks like Ensure, Pedialyte, or even Slim Fast to have WITH your meals (not in place of them). Also, Lialda is really not very effective for Crohn's, so you may not really be in any kind of remission which would explain the weight loss.
"For this thing I besought the Lord thrice that it depart from me. He said, My grace is sufficient for thee: for my strength is made perfect in weakness. Most gladly therefore will I rather glory in my infirmities; I take pleasure in infirmities, reproaches, necessities, persecutions, distresses, for when I am weak, then am I strong" 2 Cor