It sounds like Hypoglycemia to me, have a look here.
I owned a small Chi" and used to show with a friend who breed and show them. She went close to losing her best male at a show due to this problem, he just went toppling off the table sideways, out of it altogether. If your little dogs are having to cope with hot weather it may explain why it's effecting more than one. Chi' exhibitors here usually have glucose powder with them at shows in case one of their dogs gets like this. They just add water and syringe it very carefully, in small amounts, into the mouth.