My money would be on just a phase. I have seen a lot of kids go through it over the years - they've been coming from a really early age, and been fine, even through peak separation anxiety time, and then somewhere in their twos they start cracking it and becoming really upset.
My own theory is that it stems from the discovery about the same age that their actions affect the actions of those around them, and is about pushing the boundaries, coupled with an incomplete ability to categorise and express their emotions.
Try to make separations as easy as possible for your DD. Be matter-of-fact, but sympathetic about it, and label her emotions as much as possible, e.g. "I know you get sad when mummy leaves, but I have to go in 5 minutes, and I will come back after your afternoon tea." Once you have said "I have to go now." or similar, give one kiss, then GO. The more this time is prolonged, the more stressful the goodbye will be, and the longer she is likely to be upset, especially if the boundaries keep changing. "I have to go now/oh all right, one more cuddle/now are you going to be OK, I'll come back later/OK, one kiss then I have to go"
It's great that her carers have been honest with you about how long she has been upset for. If you have honest communication with them, then you should be able to work through the phase together.