My yorkie is aggressive with other dogs.
 

My dog has never been aggressive before, this behaviour happened during the winter I think because we lived in a basement apartment by ourselves, only other dogs she saw was my moms pomeranian when we visited (they're friends) and my sisters great dane (she doesn't likie the great dane at all, barks and snaps whenever it approaches her)

When I take her for walks she can walk by a dog no problem, no barking, no trying to go up to it, its when other owners allow their dogs to approach mine, lexi sniffs the dog, and then snap and does a bark. When I pull Lexi away she starts growling.

Where I'm from everyone lets their dogs run loose, and they're all big dogs, and they all come and smell Lexi and Lexi only barks at them when they hover over her, she yelps like shes hurt even though shes not.

She's 100% friendly with people, she loves people, and people love her and they all think she is the sweetest pup ever ( She turned 1 year on March 8th) Any advice or expieriences similar to mine?

Is she on leash when these off leash dogs approach her? Often, dogs feel the need to be defensive when they are restricted and others are not. We have a lot of dogs who roam loose around here too as we're in the 'country' and there's this one Lab that has charged us a few times. I had to pick Jackson up, for his safety, and he gets VERY defensive. He's not an aggressive dog and is quite dog friendly but the act of me picking him up makes him feel defenseless I think. He also does not like boisterous large dogs with no manners or good doggy language and I think it's quite normal for a little dog to protect themselves, for most DO in fact realize their size. A lot will say "oh they think they are a big dog" but really, most of the time, it's that they REALIZE they are small and that is the only way they know to protect themselves.

I would try to find a local meetup group for small dogs, where dogs are well behaved and confined to an outdoor fenced in area, etc, or an indoor, and see how she gets along with dogs in general. It doesn't sound like she's truly dog aggressive since she does get along with some dogs.

However, terriers can be prone to dog aggression more than other breeds, and sometimes right around 1-2 years old, they begin to mature and develop their dislike of other dogs. Not all dogs are going to be good 'dog park dogs'. Yorkies in general don't seem to have quite as much of an issue with dog aggression as some other terrier breeds though.

She might be reacting because she is scared! Tatiana does this with bigger dogs, if I am holding her. If she is on the ground, then she is submissive and will roll onto her back. Getting her around other dogs that are calm and will allow er to go up to the dog on her own terms might put her at ease. you would have stay close to make sure she doesn't attack!