why nightfalls

I am currently on day 11 of no pmo. But my streak is ruined by 4 nightfalls within 11 days. I am feeling weak and guilty I don't know why. I do workout, meditate ragularly and do my studies for around 5-6 hours a day. It's a busy schedule but still I have nightfalls. I not even fantasize and watched any music video on YouTube. So that I can focus on studies with pure thoughts. These nightfalls are even worse than a relapse for me. Any suggestions how to stop nightfall effectively. And I forget to mention that I got covid vaccination a month ago by covaxin. Is this a side effect of the vaccination?