Woody wrote:

After walking off 50 lbs over a couple of years I started metformin. I
didn't change my diet or exercise but gained 30 lbs back in 3 months.

This dispoves the simplistic idea that "Calories in equals calories
out". The real equation is "grams of fat into storage minus grams of
fat out of storage plus/minus water retention noise level equals weight
change".

Anywhere near the metastability of healthy metabolism the body can
change its fuel use by hundreds of calories per day to stay at a
stable size, but any change that puts a bias on the stored fat towards
accumulation will trigger weight gain. The ketonuria of low carbing is
a bias on the stored fat towards withdrawal that triggers weight loss.
(In both cases the bias happens without hunger but is easy to
interfere with, whence stalls).

Others have posted that metformin effects insulin and insulin drives fat
into storage. For some it biases them towards fat withdrawal. For you
it biased you towards fat accumulation.

When I stoped taking metformin my weight
gain stoped but those 30 lbs stayed with me so I don't think it was
watter weight gain.

When you stopped taking the pills the bias to accumulate stopped. What
did not happen was a new bias to withdraw. You can do that with
controlling your carbs, with controlling your calories, with increasing
your exercise.

A question about timing - How slowly did those 50 pounds come off when
you started walking? If it was spread out over both years that's
roughly a pound every other week. That's slow enough to be below the
noise level. You could be losing now at that rate for 6-8 weeks without
seeing it. If it was spread out over the first year that's roughly a
pound per week. That's slow enough to be below the noise level for
about 3-4 weeks. Water retention bounce of 3-4 pounds is common and
that's a big noise level. For the first month you might just need
increased patience. Not that counting your carbs or calories or kicking
up your pace a bit is going to hurt just in case ...