Hi Tim
Definitely not listened with your ear! Even with a stethoscope, some vets can't accurately interpret the level of a murmur. It would be pretty unusual to jump from a 2/3 to a 5 in a couple of months, unless there was some additional heart problem. Before you go back to your vet,
take your dog to a cardiologist for a thorough examination (you may need to ask your vet for a referral) - I can't emphasise this enough. You wouldn't expect your local doctor to treat you for a serious heart problem - he/she will refer you to a heart specialist. In the same way, 'local doctor' vets are not heart specialists. And other things than heart can cause rapid breathing - Was it a hot day? Is he overweight? Was air quality poor? Or a simple question - was he thirsty (very easy when out for the day to forget to offer water regularly)? And it isn't just increased rapid breathing that is a symptom of a worsening heart problem - you also need to look at coughing, reluctance to exercise, and a few other things. You can learn the proper way to monitor your dog's breathing rate (not just looking from the outside!) - I think there's an app you can use - someone else can tell you about it.
If you're not doing it already, there is some evidence that Omega 3 (as liquid or capsules) and Co Enzyme Q10 given daily can slow down the progression of a heart murmur. Readily available at health shops or online from somewhere like Simply Supplements.
I hope a new day has calmed your panic a bit - get that cardiologist appointment and take it from there
Kate, Oliver and Aled