out for a walk today and on 2 separate occasions, dogs mounted and humped my 19 week bitch.  I assume she is still far too young to be coming into heat, and hope this is a strange coincidence?  First time it has happened.

Its not uncommon for some dogs to be quite aggressive with pups. The only solution is to be very careful when a strange to the pup dog is approaching it, be ready to pick it up if needs be.
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Must be something in the air!! I posted something about my bitch at almost the same time!

It could just be the males, there is one dog I avoid like the plague, he is a very friend big old lab but his owner has zero recall on him and every time he sees my girl he rushes over to sniff and lick her lady part and then tries to hump her. She realy doesn't like him anymore as she finds his advances scary. His owner now avoids us too.
I don't know if he has ever been used for breeding but he seems convinced that my girl must be in season constantly.
My brothers dog who has sired two litters often when he first meets my girl gets very excited and has tryed to hump her a few times, he seems to think I've brought her down to breed but after a little while and some sniffing he settles down.