Sooo... I took Sherlock to the vet today
I took Sherlock to the vet yesterday (Visit this link if you don't know what I'm talking about, talkbudgies.com/your-budgies-health/403256-budgie-vomiting-every-few-days.html ,because this is following up to that topic)
The vet told me that there is a chance that he is overeating (although I highly doubt that is the reason for my budgie vomiting and having diarrhea), but that he most likely has a bacterial infection, she gave me some antibiotics (which I checked online, they've indeed proven useful to other people), but Sherlock doesn't want to drink 'em mixed with water, so I have to use a syringe to get the antibioticated-water in his mouth (I know how to do this properly, because my other budgie used to take antibiotics too). This is my first day actually successfully giving him the antibiotics (I was at the vet yesterday, but I couldn't find my syringe at home, so I bought a new one today). For now he is still inactive, although I can already see some changes, he's not fluffed up, although he still looks tired, and his diarrhea seems to look a bit better.
I imagine he is going to be a lot better in a week.