Another skill lost

Reagan has lost the ability to go from a sitting or laying positing to standing.  She has always done this with ease and sometimes with a little jump from the floor but it seems overnight that she can no longer motor plan well enough to make her arms and legs work together to get up and it seems to have just changed overnight.  I know that losing skills is typical of the regression stage of Rett Syndrome, which is the second stage from around 18 months to 4 years where girls lose abilities that they once had.   However, it is always really disheartening.  On a positive note, she is walking well and the falls are minimal so it has not been a big problem unless she falls and lays down close to her baby sister, who will then insist on crawling on top of her play with her nose, mouth, or hair.