I havent had it happen but i could theorize several reasons why it happened.

The most likely one (atleast the on i find most likely) is that you were relativly stressed and marijuana is the best stress killer in the world (another opinion of mine). Being constantly stressed or frequently over stressed causes the human body to produce alot of horomones that slow down weight loss. Basically our body is like "oh sheet, something bad is about to happen, we need to prepare for it." soon as it stops thinking its in for something wicked it lets you burn calories and fat easier / faster again.