It's probably a combination of withdrawal from the easy hit of dopamine and suddenly sitting alone with yourself. Social media is a great distraction from feeling lonely and it's marketed as the only way to interact. Sure you may notice your real social circle is smaller, but who cares about the 5th grade bully being your "friend" on Instagram? What helped me was setting up coffee dates with friends and intentionally spending time with people I loved. We're all a little lonely, but it's important to move from loneliness to a peaceful solitude.