Is it normal for my friend to stare at my boobs? it's a girl!

Okay so here's my story and question:
Me and this girl are pretty close friends we have a lot of history but I don't think we truly like each other, we act super nice to each other but I never feel myself around this girl she's bossy ignorant and annoying. One day we were at the beach and I haven't seen this girl since the end of school and it's half way through the summer now so I take off my coverup and apparently she notices I have boobs (which I've had for years) I guess she never realized how I have them now and everything because I usually wore sweats to school and things that weren't very showy but that day I was wearing a tankini and she was looking right at my chest I was kinda disturbed but I let it slide. At one point we were just sitting relaxing and I had my sunglasses on and she figured I wasn't looking and saw he just staring again it was so weird! She has a completely flat chest no cleavage to be found so was she jealous I don't know but then it get even weirder we are in the water and she's constantly doing boob checks like every second she's be staring at hers, she was wearing a bikini and she's pretty skinny and always complains she's fat and it annoys the heck out of me! Ugh I'm so done with this girl she is so annoying I don't mean to sound rude and mean but seriously the staring the annoying ness like ugh is the staring really normal or is this totally strange and some bizarre jealous thing?