Hi Liz
This may not help much as I am not on HRT - but I often experienced incredibly tender/sensitive Boobs about a week before my periods were due & as soon as it came and went they were fine again.. I always assumed it was due to the increase/fluctuation in hormone levels, & therefore HRT could have a similar effect in some people and nothing bad to worry about..... Am sure lots of other ladies here tho with experience on HRT, are more informed and experienced to answer you...