4 pound gain in one week?

I've been a member of MFP for about 3 weeks now. I don't have a lot of weight to lose just the pesky 5-10 extra pounds that have crept up in the last year. I had been following Weight Watchers for almost 10 years but in the past year it seemed to have stalled or I hit some sort of plateau and instead of maintaining I started gaining which is why I switched to MFP. The first week I lost about 1.5 pounds. The second week I gained back 0.5 cause I had gone to Vegas and it was hard keeping within my calories with all the buffets, etc. Just recently I've been doing a lot of circuit training. Last week I stayed within my calorie range, worked out almost everyday eating most of my exercise calories and this morning i was up 4 pounds? I know water retention, , etc. could be a factor, but is it possible to have gained 4 pounds of muscle or fat in one week? And it is not the TOM. I feel like I've been doing everything right, but discouraged right now. I've maintained a 40-50 pound weight loss for 10 years now and I know it can be slow going. I live a healthy lifestyle since finding out I have a digestive disease over a decade ago. I don't eat meat and rarely eat any fatty/ junk foods. Of course I am human and indulge every now and then (e.g, Vegas), but for the most part I eat healthy and workout regularly. Lately, counting calories, tracking WW points and exercise without results is bringing me down.