Regarding 3.5 month old not eating well

Ashley

So after my LO got vaccines we struggled for 2 weeks to get her back on track with eating - she was averaging 26-27 oz a day prior (this was 1 month ago). Then we got back for 10 days straight of her consistently taking 6 oz per feed and back to about 27-30 oz every day. Then suddenly, she won’t take more than 21-25 oz a day. At what point should I begin to worry? She’s my first so I don’t know what to expect. She does act like she’s teething (constantly drooling and chewing on anything in sight). She’s close to starting solids. She sits up pretty well while assisted and opens her mouth for anything (paci, bottle, fingers, toys). She seems to be pretty happy but I’m just worried that with her not eating as much at once that we will back slide again. She does better when we feed 4.5-5 hrs apart but then all the sudden she will fight us and only take 1-3 oz. She spits up a lot but is on nutramigen formula due to allergy issues. She’s always happy and only fussy around 4-6 pm (always been that way). Any thoughts? Thanks so much!