Is it normal for female dog to start bleeding again in third week of heat cycle?

My female English Mastiff is in heat for the first time. She started bleeding around June 23 and then about 13 days later it did get thinner and more straw colored. Now it is day 19 and she has been bleeding bright red again. Is this normal? We have her scheduled to be spayed on July 21st but was wondering if I need to take her to the vet to be checked out before then?