Is this protection?

Me, Leaim and Molly were out for a walk today, when we came across a youngish dog, probably less than a year old, now she's seen this dog multiple times in the village and has never batted an eyelid, and sometimes wanted to play with it.
The guy with the dog kept him up close to his legs as we walked up, then let them meet and greet as we got up towards them, everything was fine until the other dog went to jump up on me, at which point Molly got rather aggressive, growling, having a bit of a snap, the dog ran off without a care in the world, then we carried on talking to the other owners, and again, as soon as the dog came near me, Molly got aggressive.

This is the first time she's ever got really aggressive towards another dog, she's snapped at them if they try and get her ball at the beach, or near the river, but not in this way.

She's perfectly fine with a Labrador our friend has (About 13 months old) they've known each other since Molly was about 2.5 months old, he can jump up on me, climb on my back, etc etc, and she doesn't care.

I'm interested to see what other people think of this. Leaim seems to think it's her being protective, but it's worried me a little.

Molly was on a lead? Most dogs feel far more prtective of their owners when they are on lead. I'm certainly not saying you should let her run riot, just that this was probably the cause. She was protecting you from the 'strange' dog.

Also I have found jelousy to be a reason too

Thank you both.
She's usually always on her lead for walks, unless we can be sure there aren't other dogs around, because of instances like today, we're not really sure how she'll react to another dog.

Janey, I'd never have thought of that!

Yes, jealousy is also a BIG option!

Yes I'm afraid she loves you and is devoted to you and will protect you and is choosy who she let's jump up at you so awwww bless her she loves her family so much

Should I be stopping this behaviour?
I mean, I don't want her barking and snapping at other dogs, but I don't want her not protecting me, 6 of one and half a dozen of the other really.

That's the thing here and give her credit she recognises one dog is no threat at all and the other she not sure about. A agressive dog is straight into a fight no warning really they don't bark or growl you want to avoid that so each meeting take as a new one and as long as she can be in control then let her greet but you are the pack leader.

Good advice there, gem.

Thank you gem. I'll see what she's like next time.
Is it possible that I've caused this to some extent, when I walk her, if I see another dog that seems to be pulling to get to Molly, or barking or growling or anything, I put myself between her and the other dog. Is this right, or could it be contributing towards it at all?