Temper Tantrums in 15month old

Over the past 3 days, at 10am daily my little one has a meltdown, and cries uncontrollably for anywhere from 20 mins to an hour and 20min, there I nothing wrong physically, todays meltdown was caused by trying to get her changed. I guess this is a phase, but why are we the best tactics for dealing with this?

She sleeps 730pm to 8am, with a nap from 1pm to 3pm.

I have also noticed a decrease in her food intake over the last week or so, but I put this down to a cold although this doesn’t seem to be bothering her now. Any advice will be gratefully received.