After A Back Up I Can't Add, Delete or Edit Songs on iPhone

Last week I could add, delete and edit the songs on my iPhone via iTunes. I could drag and drop new songs, press delete etc. Now I can't.

When I right click on a song on my iPhone in iTunes, I can only play, get info (which I can no longer edit) or show in playlist.

All songs on my iPhone have a grey tick next to them, not a black one.

I normally only back up to my Mac, but for extra security I decided to back up to my PC as well. I have never backed up to my PC before. Since then I have not been able to add, delete and edit songs.

I try to select 'Manually manage Music and Videos" but I get the message

"The iPhone is synched with another iTunes Library. Do you want to erase this iPhone and Sync with this iTunes Library"?

I am getting the above message on my Mac and PC.

I know this is something simple (or rather I hope so).

I have songs on my iPhone that are not in either iTunes library so I do not want to sync with either.

I am on the latest versions of iTunes and iPhone software. I am using an iPhone 4.

Any help much appreciated. I am not very good with tech so please talk me through as many steps as you can.

Thanks in advance.

Colin