I haven't read all of the replies, so sorry if I repeat stuff, but a few thoughts ahave occured to me...

Do you have AF back, or could you be ovulating? My kids are always more fussy on the breast during those times of my cycle.

Are her nappies fairly normal? Enough wet and dirty nappies should be indication of what is going in.

My breasts always felt a bit less full and like they're not filling up so much at about the 5-6mth mark. Now, my DS2 is 10mths old and I don't even really get engorged unless he hasn't fed for about 4-5hrs depending on the time of day. It is normal for it to change.

If she is teething and a tooth is very close to erupting, she might be in pain and suckling hurts her which might cause more fussiness.

Try not to stress hun. Easier said than done I know. It all sounds normal to me. Keep up with feeding her to her demands, look after yourself, eat well, drink enough and try to rest when you can. It will sort itself out.