Being 'in my feelings' (or 'in my feels') means feeling emotional — sad, nostalgic, sentimental, sometimes wistfully romantic. 'I'm in my feelings tonight' = 'I'm a bit emotional tonight'. It was popularised by Drake's 2018 song 'In My Feelings'. If your teen says it, they are acknowledging their emotions — generally a healthy thing. Worth gently checking in if it sounds heavier than wistful.