Feeling of sand in the eyes

Hello, I'm 24F, I am taking Gentamicin eye drops and the posology is 1 drop 3 times a day. Yesterday (3rd day of treatment) I forgot the first time of the day so the next time I went with 2 drops and the next one 1 drop. Time passed and when I went to bed I started feeling like I have sand in the eyes, really annoying and can't sleep because of that. Is it because I forgot one dose, or because of the medicine? Really appreciate any help since I have lots of anxiety and can't sleep right now due to the symptoms