Do babies get weak during 9 and ten months my has gotten weak.

Not normal. I suggest immediate pediatric evaluation and testing-NOW!
No. Humans get physically stronger as they grow, and probably peak in their 20's or 30's. A child who is physically weaker than a couple of months ago, will have a medical and/or psychological reason for being weaker. The child's doctor can evaluate at the next regular check-up, or the parent can schedule an extra appointment to check things out. Regular check-ups are at age 9, 12, 15, 18, 24 months.
No, they do not. Babies do not get weak during 9 and 10 months normally. Any loss of muscle function should be evaluated by your physician as soon as possible. Please do get an appointment as soon as possible!