Hi K.,
L. the lactation consultant here. With her pursed lips, and behavior, your baby is communicating to you in the only way she knows how that she does not want the bottle (for wahtever reason). Maybe her gums hurt, maybe she has reflux, maybe her ears hurt, maybe someone forced the bottle into her mouth, maybe she just doesn't feel like eating. She's showing you she's not receptive to the bottle or its contents for some reason.
As you said in your post, it's best not to force the bottle into her mouth - that can create an oral aversion. Do some detective work and ask yourself, "Has anything in her life changed significantly in the past few weeks?" If she's receiving formula, have you changed brands, are you mixing it with the same water as before, have you stopped heating it (if you were in the past), did someone else force the bottle into her mouth? Have you tried a sippy cup?
You've received some great advice from others and it is true that weight gain slows significantly in the second six months of life. The "typical" baby has a weight gain of 4-7 ounces a week during the first month; an average of 1-2 pounds per month for the first six months; and an average of one pound per month from six months to one year.
Solid foods are generally started around the time baby is six months old.
Teething could be an issue, as could an ear infection, GERD, or other medical conditions.
Check in again with your pediatrician, explaining how difficult giving the bottle is for you and your baby. Or consider getting an opinion from a different pedi. In the mean time, do what works. A thorough medical evaluation is in order, in my estimation.
Please keep us posted on her progress.