It's happened to me too! I was feeling really slim and was fluctating between 138 and 139 (a 1-2Ib loss for this week.) Today after I went running I ballooned back to 140
I feel very bloated too. I'm hoping it's muscle gain and not stress related water retention. Tomorrows my weigh in day as well, it's going to look like I lost nothing this week so I'll have to rely on inches to see if anything's changed.