Ryan, this all seems a bit strange to me! Firstly the laminitis. Yes, some mares do develop a sudden attack of laminitis very late in their pregnancy or even over foaling, but you can usually see it coming from the way the horse is moving. A couple of days ago you took Rexina for a walk and commented upon her being a bit crabby and biting at her stomach, but you said nothing about her 'shuffling' along on possibly painful feet?? When out in her paddock has she been rooted to the spot (severe laminitis) or moving around grazing normally (allowing for the normal slow movement of a mare in late pregnancy)? However, if she has indeed aborted/lost the foal this late in pregnancy then that cold bring on an attack as any stress can cause laminitis.
Did this vet give you any comprehensive instructions on how to deal with the laminitis - stabling on deep soft bedding to encourage her to lay down/rest and also to support her feet - treatment for her feet (hosing etc) - a feeding regime (as Jax says to soak her hay will help to reduce the suger level even further), being careful that the sudden change in diet doesnt cause colic, keep your hay soft and leafy - stemmy hay could well cause a blockage and/or severe colic - soaking the hay will also get some fluid into her as coming off the grass will reduce her liquid intake. Dont forget that she can still have, and needs, her mineral and vitamin supplement/additive even if she is laminitic.
It is of course possible that she is having a false pregnancy, but your usual vet was almost 100% sure of the pregnancy back in February so must have felt something and I'm sure that you would have noticed if she had lost the foal between then and now. But maybe the vet was mistaken? Then there is the change in the colour of her vulva ................................
As I said, it is all most strange and i really dont have any answers for you, it is sometimes very difficult over internet posts and pictures to get an accurate picture of what is going on. Perhaps you should try and get your original vet out again to check on her - anyway I expect they will want to keep checks ongoing due to the laminitis.
Just remembered that Holly is having problems with her maiden mare Indy on here - not due for sometime she keeps developing an udder, another strange occurance!
Wishing you and Rexina good luck - how about some more pictures for us?