It's not impossible, but it would require astounding fertility on your part. If sperm comes into contact with the enzymes excreted in her vaginal fluid they will travel towards the ovum. The amount of your sperm that's going to make it that far through 2 layers of cotton is pretty minuscule. In normal sex, 90% of a man's sperm dies off swimming less than an inch from his ejaculation to her ovum. The chances that a small portion of your sperm could travel the whole length of her vaginal canal and still have enough strength to break into the ovum and impregnate her would be a miracle.
If you're worried about it, have her take a home pregnancy test. They're inexpensive and very accurate.