as your mum went through menopause so early i would imagine she's been on hrt ever since, so maybe she's just come off that? if she has, then she is in effect going through another menopause as the hormones she's been taking are withdrawn and her body has to get used to functioning without them.
some ladies find these symptoms (there are 35 well documented and troublesome symptoms, one of which is pms type mood swings) take years and years to diminish, and some poor souls never get rid of them, although most do. i would say the averge is about 5 years or so.
do some googling about menopause and you'll come up with a lot of helpful information for her (including these 35 symptoms, many of which you would never even associate with menopause and which she may be having).
tell her good luck from me, she has my sympathy.