Along with the other suggestions...
You have irritation from something...
With her winter coat has her halter become to snug?
Has she lost weight that now her halter is to loose?
Has the nylon halter been cleaned, truly cleaned lately?
If not, hot soapy water and some scrubbing with a anti-bacterial soap, rinse well...hang to dry completely.
I know some who have put their nylon halters in the dishwasher to deep clean.
If leather...hot soapy water, anti-bacterial soap to scrub the entire halter then repeat with a appropriate leather cleanser...let thoroughly dry then if needed/warranted condition that leather to nourish what that harsher soap stripped off and out of the leather.
I would also look very carefully for hidden bugs or "friends" to our horses such as lice...ick.
Tis the season when itch and hair are making everyone rub, scratch and dig using anything the horse can utilize as a scrub post..
Ticks are also emerging again...search very carefully.