Kel Mitchell is finally explaining what the heck landed him in the hospital -- and it sounds like he feared the worst for a quick minute there ... with some truly scary symptoms.

The former Nickelodeon star hopped on IG and showed everybody that he's doing much better now and in good spirits -- not to mention spilling on what everyone's been wondering since he first went into the ER ... namely, WHAT HAPPENED?

According to Kel, it all started Tuesday when he was walking around and started to feel dizzy -- which he says he attributed to him possibly being dehydrated or simply low on food.

However, when parts of his body started to go numb ... that's when Kel says he started panicking and got rushed over to a hospital. KM goes on to say he couldn't lift his arm for a bit -- and that he and his loved ones suspected something terrible had happened. He doesn't say it explicitly ... but it sounds like he might've thought stroke, considering the warning signs.

Fortunately for everyone, it wasn't that ... as it turns out, Kel says an old injury flared up and that caused a bulging disc along his spine to rub up against a nerve -- and that was causing the problems he was experiencing. TMZ broke the story ... he came in conscious and alert.

Kel says they ran a battery of tests on him, which is why he had to stay overnight ... but he's out now and doing much better. The guy again thanks his fans for the well-wishes.