For me, while there's been many times, most times I didn't know how to describe what was going on, when I would lose skills, and mostly believed it when I was told that I was being "unco-operative" or "playing games".
It is quite common for autistic people to have atypical acquisition and loss of skills, and it can also be triggered by overly stressful events, like change in environment or other physical or emotional distress.
I used to be really good with rote memory and arithmetic, and also pretty good with abstract math reasoning, though difficulty with understanding instructions and sequencing made me start out at each math lesson in elementary school as the slowest to learn, but once learned then I would be tutoring others on those same concepts.
I taught myself algebra and calculus when at 10, then I forgot most of it, then I learned it again at 13, along with physics, then forgot most of the calculus, and then learned it again between ages 16 and 17. At age 16, I picked up a textbook about number theory, as well as one about modern algebra.
This was in Algebra II/Trig, and I had for two weeks the ability to instantly solve logarithms problems, that would take an overhead sheet or two and half an hour for the class to solve. A few weeks later, and I couldn't remember any formulas for the next test, and got a C-.
In the two years following that, I've gotten especially good with conceptual math, and can understand the formulas so long as I don't have to remember them or to calculate arithmetic (advanced math is mostly proofs and theorems anyway, so that's no big deal).
I have had varying skills with speech. It varies more on a day-to-day or minute-to-minute basis, though, rather than between months. Though I can usually speak, I usually have to have a lot of time not speaking, not in crowds, stressful things like that. Otherwise, speech will shut down.
Since at school I am around lots of noisy kids, crowds, processing tons of speech, fluorescent lights, having to keep attention to tasks, socializing - it's a heck of a lot more stressful than a day at home with my at-home routine of court shows, I Love Lucy, Tetris, swimming, walking, writing, reading, and Internet. So speech shuts down a lot more often for me when going to school and other such busy places, which tend to put many more demands on me than the non-pressured summer-at-home environment.
More recently, in high school, while overall I was gaining a lot of skills (body awareness, identifying and articulating feelings and sensations, initiating things, socializing, riding the bus independently, etc.), the skills that I lost (reduction in how often speech is possible, arithmetic, increased rocking and other stims, generally being more visibly autistic) apparently caught their notice, even though things like stims enabled me to learn the other things and did not give me trouble.
They also said I was having more frequent meltdowns and shutdowns, but any consultation of my mother regarding driving me to school and picking me up four years earlier, would have roughly disabused them of this notion - my looking more visibly autistic clouded them to think I had increased meltdowns.