As of now since you are breast feeding let's not give you any pain killer medicines let's try to keep you treated conservativly with a collar neck support and use a very soft pillow when sleeping... Try not to see your mobile or read books in sleeping position.. Sleep on your back with neck straight 20 mints twice daily.. As for your pain sternum try topical voveran gel. Your giddiness will stop once to start wearing your color neck...
Next Steps
If it doesn't resolve try to consult a orthopaedic doctor for MRI and further expertise
Health Tips
You should be OK with collar neck support worn only when you are awake. Remove it on sleeping..