I recently got a 2-yr old shih tzu about two weeks ago. She's really sweet. She rarely barks and never bites and is always easy to manage. From about a week ago, she acts strange with me. This happens abruptly. She'll be fine and the next thing you know, she's sniffing me and gets scared/anxious (well, at least I think she is). She doesn't do it to any of my other family members. ONLY me. I thought it could have been that she knows who the youngest is (I am. I have an older sister and my parents). When my acquaintance's little kids came (age 4 and 6), she did the same to the younger one.
She would try to sniff more of my neck, chest, arm area. She whimpers and whines and looks at me and comes back to me and try to sniff me some more. She stands up and try to reach my shoulders so she would end up scratching me a lot of the times. Then she seems to get scared/anxious and just circle around me and eventually just strays away from me. Her tail is constantly wagging though.
I thought it could have been my sweat smell since I do work out a lot. But she does that even after I take a shower. And lately she's been doing more and more and I have no idea why she does this.

It sounds like she LIKES you more than the others, and is anxious for some hugs and kisses! Dogs like to get close to your face when they like you.

It sounds to me that she senses that you feel like you're the bottom of the pack, because of your age. Since she's still young, you've become a playmate, someone on her level in the pack. It may be that she's trying to one up you in the pack, become higher in the pecking order.

It also could be because you're obviously reacting to how she reacts to you. You feel stressed that she's doing it, and she senses that, and reacts accordingly, by showing fear.

In other words, she's anxious because you are.