why do i feel this " sinking feeling " when my guy friend befriends another girl, but i don't crush on him ?

i have a guy friend and he's getting closer with his new female friend and they even tagged each other on Instagram. but, whenever i see them hanging out together, i feel as if my heart is " dropping " and " sinking ", and i keep thinking about it throughout the day to the point where i can't even focus on studying. but, i don't have a crush on him. im a girl who believes that staying single has way more benefits than dating a guy. but, i just don't know why i feel this way. but, i don't feel this way when he's with his male friends. i only feel this way when he's with that girl. why am i feeling this way when i don't crush on him ?