Well without knowing how you've put your opamp circuit together how can we guess what's wrong.
Since you have a strain guage you will clearly do best with an instrumentation-amplifier setup - some are available on a single chip, there's a standard way to fabricate a high-performance instrumentation-amplifier from 3 op-amps too. A less performing solution with a single op-amp is the standard differential amplifier circuit. All of these you'll find on wikipedia these days, BTW.
What accuracy are you looking for BTW, that affects circuit design...
The LM324 will work from 5V single supply but you must keep all the signals in the 0 to 3.5V range, and best to keep them in 1.5 to 3.5V range.