I was feeling my eyelids and noticed that under my left lid there's a very tiny bump, feels like the size of a grain of sand. what could this be?

Most likely a stye: A stye is a tiny gland in the eyelid which gets blocked off and begins to swell. They are generally harmless, but may get infected (red, swollen, painful). A stye will often go away if a warm compress (washcloth soaked in hot water) is applied for 10-15 minutes 4 times daily for 1-2 weeks. If it is not improving, or if it gets worse, see your primary care doctor or an eye specialist.