A single strip gives you very limited information. As does an inverted T-wave alone.
Before basing anything on a single rhythm strip make sure the placement of your leads are correct. If the leads are placed differently from standard positions to get the best visible QRS-complex for example, your inverted T-waves could simply be the result of non-standard lead placement.
But in general if your T-top was positive to begin with on your single strip and then turned negative something is up. But since the causes of negative T-waves are numerable I'd suggest getting a 12-lead EKG when you see your T-waves change, it's your best bet to find what's causing it.
Since your T-wave represents your repolarisation, changes in it have to do with changes in repolarisation. And there is a plethora of causes for repolarisation disturbances. The ones you'd have to keep in the back of your head are ischemia/infarction, hypokalemia, toxicity and PE. But without a 12-lead EKG or previous strips to compare to with verified lead placement, an isolated find of an inverted T-wave might mean nothing at all.