my female becoming over excited when tied

Question
My female came into her second heat, she first tied with my male on day 6 of her cycle. It scared them both at first and then they were ok the next day when they tied, they've  tied once a day for 7 days now and the past 3 days she has become increasing overexcited once the tie doing circles, jumping and trying to get away, I have to sit and hold her still trying to calm her until the relase. Is this normal behavior, can either of them hurt each other, is she coming out of heat, and if so why is she standing for him and flagging him then behaving this way. Also how long will they continue to breed if she is already pregnate?

Answer
Hi Tracey:

Dogs will continue to mate as long as the female is in estrus, which is the fertile portion of the heat cycle. On average it lasts for approximately 4 to 12 days. Since they've already had more than enough successful matings I would urge you to keep them separated for about a week so that hopefully she'll then be out of estrus & will no longer allow mating to occur. Her actions are perfectly normal but could cause injury to herself or the male if allowed to continue. When breeding occurs, the male's penis swells & puts pressure on the vaginal tissue, often causing discomfort & even pain. She is simply reacting to this. She may have been more tolerant in the earlier part since instinct tells her to cooperate but now, since she's probably in the latter stages of estrus, she's simply being less so, although still allowing mating.