As noted earlier you could be young for frozen shoulder.
With regard to diagnosis, there can be many possibilities based on the area and restriction of movement.
However generally, icing for 15 mins with a gel pack/ice wrapped in a towel upto 3 times a day during the initial period helps.
Next Steps
Icing and watch my video for shoulder basic mobilisation on YouTube.https://www.youtube.com/channel/UCl1cWua2RplXZcbLO6hHdFw
these are basic exercises.
You need a physiotherapist consultation to diagnose your condition and treat it.
Health Tips
Do not lift heavy objects at this stage