Her doubt of their relationship feels kinda forced. Like, they're 16 and in their first supposed relationship, and they're like "so embarrassing!" after she introduces herself, or she hesitates for a moment after being asked to list off all the things she likes about her boyfriend to said boyfriend's mom, and mom is instantly "HUH, WHY ARE YOU EMBARRASSED ARE YOU MAYBE NOT DATING AT ALL?!" It's like... That's 100% normal behavior even if you are dating, lol. But the joke in this chapter requires her to doubt their relationship, so she gets suspicious even though it doesn't make sense at all.