Could I have possibly gotten burned doing this!?!?

[mutantfrog]

OK.
Question regarding doxycycline and sun exposure. I know there have been a few of these...

I took doxy for a long time when I was a teenager and early 20's. Never was too much more sensitive to the sun while on it (same dose I'm on now).

Took a break because of some allergy confusion...and now have been back on for about 2-3mo.

I do put on sunscreen on my arms (usually some kind of Coppertone, SPF 50), chest, etc now that it's warm outside and I'm wearing tank tops.

I didn't put any on Sunday because I was literally going from my house to the car (15sec), traveling in the car 20min, and then again walking from the car to the studio (15sec). That was the total sun exposure.

I somehow got burned really badly on parts of my arm - like the shoulder cap, then a big gap, and the lower part of my arm (right above the elbow) and upper part of my forearm. Only on the left side - so I am assuming it's from sun while I was in the car?? Except that i thought I was protected?? Now I'm just really confused. Windows were up the whole ride.

So here are the questions:

-I was told (by a derm) that car windows protect from UV. Is this true?
-has anyone else burned this intensely from such a minimal sun exposure??? I mean, either it was through the window somehow driving (in which case, 20min isn't too minimal I guess) or the few seconds walking to and from the car total that day.

Sorry I'm just trying to figure out if somehow I'm indeed not protected in the car, or if the exposure necessary to burn now while on doxy is really that small. Wasn't an issue for me before, so i was really taken by surprise. I thought I had like a cellulitis type infection at first - not realizing it was just sunburn.

Would appreciate any advice or experiences...
Thanks!