I have a friend whose first son had 'low muscle tone'. I live in Scotland and she's in the States, so I don't know all of the ins and outs of when he did what, etc., but I do know that he was quite late in walking - close to two years old before he was walking on his own.
When I last saw him, he was 28 months old and still quite clumsy, etc. on his feet...I think perhaps the low muscle tone diagnosis came because of delays in holding up his head, sitting, etc.
He has received intensive physical therapy for over 2 years (he's 3+ now), and he's doing well. Very sweet, well adjusted, intelligent child. He is still a bit on the slow side physically, and will probably never be a star athlete, but he's getting there.
Clarity has given you good advice...I would have your daughter evaluated and get her into physical therapy. At 8 months, she should be holding up her head.
I know it's scary, but it's amazing what physical therapy can do for kids.