A physiological problem due to extreme temperatures, be that high or low.

See http://www.ipm.iastate.edu/ipm/hortnews/1997/7-18-1997/tomdis.html

Here is a link that might be useful: tomato woes

Puffy tomatoes---thanks :-)

From your link,

Cause: Extreme high or low temperatures, excessive nitrogen fertilization, and heavy rains may interfere with normal pollination, resulting in puffy fruit. Puffiness occurs most frequently on early fruit. Control: No effective controls. Puffiness should decline later in the summer.

Here, in my zone 6B garden, the temp extremes were never hit---nights were over 55 and days were under 100

I had watered more religiously than in previous years when I had grown Mule Team but it was always watered at the base of the plant so I'm guessing this shouldn't have interfered with pollination?

I had used a small dose of 5-10-10 early in the season, which I had discounted as a cause. Found this study by Dept Agriculture (38 BULLETIN 859, U. S. DEPARTMENT OF AGRICULTURE) where they tried to determine if a fertilizer inbalance was the cause:

Various fertilizer plats were arranged to determine the effect of
different amounts of nitrogen, potash, and phosphoric acid upon the
production of "puffy" fruit. .....

No positive results were obtained in this study showing the cause
of puffiness in tomatoes

Puffiness may therefore be dependent upon an unbalanced soil solution, but,
if so, none of the variations in the fertilizers just enumerated sufficed
to restore a proper condition. It is, of course, not inconceivable that
puffiness is of a genetical nature and due to somatic variation. If so, it
might, in conformity with the observed facts, be much more frequent
in some varieties than in others, and the same plant might show both
normal and "puffy" fruit. The whole subject is one which needs
investigation.

I think I'll get a soil test next Spring and find a "new"favorite tomato to grow.

Thanks so much for the help! :-)