Sounds like a hot spot. My dog had one last year on his side.
Reminds me of when my 2nd whippet was still fertile, before I had her spayed. I think she had 2 or 3 heats, and they were looooong. You can buy "bitches' britches" to hold a sanitary pad, but with the tail, it's hard to get that pad in just the right position, so it's still hit or miss (literally). Plus a whippet has such a sloped back and hindquarters that most doggie pants don't fit. I bought little boy briefs and stuck the pad inside and pulled her tail through the fly.
Keiki's Makuahine (Keiki's Mom) 50, dh 50
Keiki: b. 2002 after 3 months bedrest
Natural conception following ZIFT/chem. pg
Olivia: b. 1999 d. 1999
28-week preemie, ptl cause unknown
Natural conception after 1 mc
ttc since 1998