Why do i get nervous when my girlfriend touches me below the belt?

okay so ive been dating this girl for almost a month and i REALLY like her alotttttt. like shes the best girlfriend ive ever had by far because we connnect so well and can talk anytime about anything. all we've done is makeout pretty much, but she rlly likes to rub my upper thighs..like she'll get under my shorts and rub my legs as high as where my boxers are. n when we makeout i alwayss get a hard on..idk if thats normal either cuz its just kissing.. so let me now. but liek today we were making out and she was on top of me, and she came VERY close to touchin my . and for some reason i got rlly nervous. i think its because im self concious or worried she wont think im big enough. any suggestions or comments? =] thank u guys