No weight gain in 9 mos old

At her 6 mos apptmnt, my daughter weighed in at 18lbs, 2oz. At her 9 mos
apptmnt, she weighed in at 18lbs, 3 oz.

The ped's suggesting I supplement her with a sippy cup of formula/day.

I'm concerned that there wasn't just a slowing of weight gain, but a
complete stop. Now, my daughter has been sick over the past 3 months,
but I don't think that's explanation enough. She still nurses a lot,
including every 1.5 - 2 hours through the night. (My ped. suggests that
could be because she's just not getting enough milk from me anymore.) We
nurse one side at a time. Rarely does my daughter fuss after finishing
on one side. When she does, I flip her to the other side and we're fine.
We're still getting good wet & dirty diapers, I'm not at all worried
about dehydration.

My child has become very active. If she's not moving, she's not happy.
She wakes herself up at night because she's crawling & standing in her
sleep. She's not shy about crawling over to me and having a sip or two
and then going back to play.

She eats everything we put in front of her, but small amounts. I don't
think she's ever eaten more than 6 ounces of table/babyfood total in a
day. She still only drinks 4 ounce bottles at daycare. Every time I
think of increasing it to 5 ounces, she has a week where she'll leave an
ounce or two in every bottle.

Any suggestions on what I can do to make sure she has all the food she
needs? Does the collective brain here see any benefit to giving her some
formula?

I'm to go back in 2 weeks for a weight check. If she hasn't gained
weight, the ped's going to want to do some testing to make sure there
isn't some other problem going on. So I'm hoping for the weight gain.

Thanks for any advice.
Leigh