Jess,

Good perception and situational awareness is something that when exercised properly can change situations.  You did great moving away from the potential threat an BETTER YET you did an assessment of your actions and the outcome and concluded correctly that your actions prevented a potentially disastrous situation.  Great job keeping you and your dog safe.  Did you talk to the owner of the Bullmastiffs? 

Maybe you can chat with the owner and get some photos. :)