Why are woman embarred about their period? ?

My gf one time at work came to the back room to get a tampon out of her purse. and i was in the back reading something i think, i was like "hey whatcha doing" she went ALL RED and was kinda pissed made a weird face and snaped "What do you think" and waved the tampon. And its not like i didnt already know she was on her period, i mean, its close to the same time as last month... and she had a purse with her, which she NEVER does unless she is on her period. Now i can understand, its not a very pleasant thing to deal with so your bf being there might make things a little edgy is alright, and she probly didnt know i was there so its like i kinda ambushed her but, even when she isnt on her period or many woman for that mater, they are really snappy and easily embarred about it... its natural what the hell is your prob?!?! Lighten up, and not all guys are embarred about it too, or bras.... (just fyi, cause all the girls i work with think that they cant talk about ANYTHING around be, im like why not, cause they think ill get "squemish about it")