5 month old not feeding properly

[SuddenArborealStop]

My DD, 19 weeks on Monday, is driving me up the wall fussing off her bottle. She managed less than 20oz total yesterday and used to take 30.
She is a very very snotty baby and literally sounds like she's drowning in her own fluids sometimes so I try to give her nose drops before starting the feed so she can breathe. She's teething which makes her blocked nose worse but also means she's chewing the teet instead of sucking.
She had the rota virus vaccine last Friday and feeding seems much worse since then but it could be coincidence.
What do I do she's really worrying me. I tried a sippy cup this morning and still she wouldn't take it.
I was going to start weaning soon so I could bring it forward but she's still going to need mostly formula.

Bumping, still not taking more than three oz every three hours, even after sleeping 12 hours overnight.

Mine was the same after the rv vaccine, got him back upto 30 oz then the last two weeks it back to fighting to get 20oz in him.

No advice I'm afraid as I cry daily over it but all you can do is offer smaller feeds often minimise distractions and try not to show your upset to the baby.

I'm assuming they will eat when hungry and won't starve themselves.

I try teething powder before a feed and that sometimes get more in.

Out of interest how many feeds is she having a day?

My DS always started to refuse to drink more than 3-4oz when he was trying to consolidate bottles. So if he’s was having 6, I’d stretch the gap between bottles and go down to 5 and it used to do the trick

Timing of the feeds doesn't make too much difference to be honest. She's back up to 25 oz now two weeks after the vaccine, my first reacted badly to it too just not this bad.
Her cough has gotten worse though despite sterimar at timed intervals to keep her clear. So disappointed there's nothing the docs can do as she seems to stop breathing for a second at the worst bit of it. this scares my DM so she won't take her for me and after five months on 24/7 care in lockdown I was hoping for a few hours off.