I lifted weights Monday. Yesterday morning my weight dropped to 137.6. Then this morning it shut up to 138.2. Is it possible my weight is being affected by the weight lifting I did Monday (48 hours later)?

It is possible. Our muscles tend to retain water as part of the healing process so its not unusual to see water weight on the scale.

And we all know water retention means nothing.

I've gotten a 3 pound difference from water alone!

Absolutely. I can see a 4-6 pound swing from a heavy lifting day, it can take a few days to settle back out.

I know I am late in this but also, it is quite likely that 0.6 lbs is well within the error tolerance of your scale as well.

I am very happy to see these postings. I've been on WW for 45 days now and weighed 267 Saturday morning. Today, after my workout, I weighed 273! I was so-o-o bummed.