Weight Issues

I've always been thin.  I mostly attribute it to genes.  My mom was thin well into her 30's and my dad is still tall and lanky.
You've read that I gained 50 pounds while carrying Lauryn.  Needless to say, I freaked out.  It's not as if I was eating poorly or not exercising.  Though I was miserable carrying it, my biggest concern was losing it. 
Well, 3 months post partum I was back to my pre pregnancy weight.  My body didn't look the same (and, I am okay with that), but I was able to fit into my clothes (one of the things I missed most during pregnancy).
However, as the months progressed, the weight continued to drop
and drop
and drop.
I began to worry.  It was noticeable.  My clothes became baggy and people began to make comments.  I scheduled a visit with my primary care physician who did bloodwork.  My thyroid was "borderline" (something I was made aware of in college), but nothing of concern.
The weight continued to drop
and drop
and drop.
I didn't like the way I looked.  Everyone tried to reassure me that it was because of the breastfeeding, but I wasn't so sure.  The weight loss had been gradual, then all of sudden dramatic.  I continued to monitor it for a couple of months, but eventually wanted a second opinion.
I visited my OB this time.  I immediately felt better because she was also concerned.  Everyone else dismissed me with comments like, "that's a good problem to have."  She took me seriously.  My charts showed that I'd lost 18% of my body weight since August.  18%!  She ordered extensive bloodwork and told me that a "borderline" thyroid was not okay being that I've recently given birth.
My results are back.  I'm okay.  Again, nothing of concern.
I feel more comfortable with these results.  I also think I feel better now that I am weaning and pregnant.  Weight gain is emminent.  Hopefully, I won't end up on the other end of the spectrum (gaining too much) again.