You could also view it as a compliment. The person notices that your hair is longer so they ask if you're growing your hair out (from your last cut/trim).
So,
are you? I would like to know now that you started the thread.
In real life, I tell people "Oh, I'm happy at this length" - people ask me about my hair if I wear a twist out for three months and then one day they see me with straight hair. It's a drastic change (shocking for some), from above shoulder length to about bra-strap length.
They also ask me "Did you lose weight?" (The answer is no, but I am wearing slim fit pants instead of boot cut!) I usually just smile and ask them if they are toning up for the summer. (lol)
Girl - people just don't know what to say, so they say anything! Silence is golden.