Hi there and welcome to bw. Im sorry you are feeling so worried. At 5 months 4 hrs between feeds should be fine, does she appear really really hungry at 3 hrs? Just you mentioned it can be every 3-4 hrs she eats. It may be she is snacking, and if you could stretch the feeds to four hourly all the time she would eat bigger quantities. Also, I know you briefly outlined her behaviour whilst feeding, but can I ask you.
does she spit up a lot?
Arch her back during feedings?
Fuss and scream as if in pain?
Seem in pain when laying down?
The reason I ask is I was thinking of a possibility of reflux. Even silent reflux when they do not vomit can be painful as the acid backs up their throat.
Also, what does the doctor think of the slow weight gain? 18-23 is within the normal range for some babies, as some babies just do not drink massive quantities of milk.
When did this behaviour start?
Is it something that has gradually worsened?
Sorry for all the q's but the bigger the picture we can build the more advice you can get
hugs, I know its frustrating I have so been there and used to desperately post on here as well.
xxxx