Hiya - hope you aren't feeling too achy at the moment!
It seems to be quite normal for measurements to be over or under from my and friends experiences. Some of my friends that are currently pregnant are measuring spot on, whereas others are measuring up to 3 weeks bigger. Apparently I am 1/2 a week bigger than my EDD - god knows how you can measure 1/2 a week, but that's what the midwife says!! hee hee
If she was concerned she would have sent you for a scan etc. I'd look on the positive - the baby might be a small one!!
As for breathlessness, well, I'm 30 weeks and at work i'm constantly huffing and puffing as I walk up the very short corridor! It's just where the baby grows and your lung capacity gets smaller due to baby being in the way. Aching is quite normal too. Everything squeezed up and out, more weight to carry in front so over compensating by positioning your body differently, increased hormones, baby taking nutrients from your body - it all adds up to one thing - aches! (unfortunately) Also, the baby will be beginning to dip into the pelvis and 'engaging' so that would explain the heaviness.
BUT - if you are concerned and the pain / ache gets worse, defo see your doc / midwife to double check. Braxton hicks could also be a cause.