This happened to me a few years ago. As pp mentioned, it was chronic idiopathic Uticaria with angioedema (the mouth/eye/tongue swelling) - basically unexplained hives. It was exactly as you are describing and came out of nowhere - and it wasn’t an allergy. It would disappear and then randomly I would be covered again. It got worse and worse as the days went on, and eventually I would be covered from head to toe several times a day. I had a scary trip to A&E because my tongue swelled up and on another occasion my husband had to call 999 because my throat started to close. I had a battery of blood tests but they never found a cause for mine (sometimes it can be thyroid or hormone related, or the after affect of a virus as mentioned before) it went away randomly over the first lockdown, but I was taking a very high dose of antihistamine plus some other drugs that are known to be effective (one is for asthma, the other is stomach related, I can’t remember the names!)
be careful with the steroids because they do work but they aren’t a long term fix, and they are what I was prescribed by A&E and the GP before I was diagnosed because that’s all they know that will stop the hives. You need her to see an allergy specialist that deals with immunology - it isn’t an allergy but it comes under this remit. If you can go private I would really consider it, just to get a fast diagnosis - because honestly you could go round the houses looking for an allergy when this is so typical of chronic idiopathic urticaria but is not something GPs are very familiar with (in my experience). My GP was very reluctant to prescribe a high dose antihistamine but the allergy specialist wasn’t, and the GP would never have prescribed the other combination of medication that I was on.
It’s a horrible condition and can be very scary - I was told to avoid any kind of ibuprofen type drug as this can exacerbate it (I was told this after I had taken ibuprofen and my throat started to close) I can now take ibuprofen again, it seems to only be an issue when the condition is ‘active’.
If you are near London I am happy to recommend the clinic I went to - (they transferred me over to the nHs list once I had been diagnosed, and it only took 1 appointment for diagnosis). There’s a few threads on here that I found useful, if you do a search (that’s how I figured out what it was!) and there’s a very helpful Facebook group as well. I really sympathise with her not wanting to go out, I absolutely hated it when I had a flare up on my face and had to go anywhere - once my medication was sorted that never happened again, there is hope!