Occasional aggression towards dogs

[Evahill]

Hi,

I have a 5month old border collie pup called Cleo and she’s brilliant, loves peoples, picks up commands well and is just great.

However, with dogs she very hot and cold. One minute she’s fine, when on the lead she’ll have a sniff and we’ll walk off. She has a couple dogs she knows well now and plays with happily. Other times she switches and starts growling and teeth showing. She uses her paws a lot which makes it look worse however I have no idea where this comes from. If she’s on the lead and growls when a dog approaches I think it’s out of frustration as when she’s let off leash she is fine. 
 

Although sometimes off leash she will growl and show teeth and play rough. She’ll stop and start kinda thing but it looks bad like she’s not playing. 

I’m worried it’s aggression, but I can’t pinpoint a reason as it’s so random as sometimes she’s fine sometimes she isn’t .

When she was very young she was very timid around dogs and shyed away so don’t know if it’s maybe out of fear or intimidation. I want to tackle this as don’t want her thinking this behaviour is okay. 
 

Any tips or help would be great.

Thanks