Most people who wake up from Deep Sleep, and then go back to sleep, do not remember waking up at all. For a period of several minutes after waking up from Deep Sleep, we experience sleep inertia, characterized by lowered cognitive abilities and a reduction in the ability to make memories. This same effect is why people feel more groggy when they wake up from Deep sleep as opposed to REM sleep.
That may explain why you don't remember waking up, but not what is waking you up in the first place. It's possible that a snoring partner - and other household noises - are disturbing your sleep. Try using earplugs or installing a white noise machine to see if this makes a difference in your disturbed sleep. Many other factors could be at play as well, such as light at night, substances taken during the day such as caffeine, or even the comfort of your mattress.
You may also want to ask your partner if you snore a lot or ever wake up gasping for breath, both of which can be indicators of a sleep condition known as sleep apnea. Feeling tired and fatigued during the day is another warning sign of this potentially dangerous sleep condition. Most people who have sleep apnea do not realize they have it until confronted with the evidence from a partner or a sleep doctor. If you'd like to find out if you may be at risk for sleep apnea, you can take this quick questionnaire here.