I'm having exactly the same problem and was actually googling to see if it was normal I am also 21 weeks, had morning sickness since 6 weeks (only threw up a couple of times but constant nausea). Nausea went at around 18 weeks but is now back. I haven't been sick this time but have been constantly nauseous for the last 3 days.
I don't have a temperature so don't think it can be a bug and eating helps, (for the short time it takes until my stomach is empty again ). I'm feeling tired and yuk.
I hope we both feel better soon, I thought this was supposed to be the comfortable, glowing, energised trimester!
If you find any miraculous cures please let me know and good luck.
I sympathise. I am 19 weeks and although the nausea of early pregnancy has disappeared (although I wasn't actually sick until about 10 or so weeks) I throw up like clockwork most mornings. I then eat breakfast and it's fine but no matter what I do I wake up retching.
I've kind of resigned myself to it sticking around for the duration now tbh.