Apple Watch glows red from the underside. What does that indicate?

See the 'How Apple Watch measures your heart rate' section (especially the figure) in:

Monitor your heart rate with Apple Watch - Apple Support

The optical heart sensor can also use infrared light. This mode is what Apple Watch uses when it measures your heart rate in the background, and for heart rate notifications. Apple Watch uses green LED lights to measure your heart rate during workouts and Breathe sessions, and to calculate walking average and Heart Rate Variability (HRV).

Is it normal for the underside of my watch to light up occasionally while I’m wearing it in bed, it’s kinda bright when your arm is near when your on your side?

there is a red light underneath my Apple watch that lights up when I am in bed. Why?