It's perfectly normal behaviour, especially for young puppies of either sex.

Our 5 month old girl still does it occasionally, although no longer ot her bed, after being asked not to. Instead she'll now occasionally hump our older bitch! Older girl doesn't seemt omind at all, and the behavioru itself is harmless. They normallly grow out of it when their hormones even out, but it can occasionally become a learned behaviour. (less often a learned behaviour in girls than boys)

I prefer her not to do it though, so I usually just tell her no whenever I see it, which stops it. I assume once she gets older, the older girl will tell her off herself for doing it to her!