I used this ladies credit card to buy a $1 pregnancy test (without permission) while I was doing her shopping but I returned the money. Now she is claiming it is theft. I was wanting to know if it is so?
Depends where you are
At common law, theft (or more generally, larceny) requires an intent to permanently deprive the owner of possession.
118 Intent to return property no defence
Where, on the trial of a person for larceny, it appears that the accused appropriated the property in question to the accused's own use, or for the accused's own benefit, or that of another, but intended eventually to restore the same, or in the case of money to return an equivalent amount, such person shall not by reason only thereof be entitled to acquittal.
In Germany, the situation is similar to the one under Common Law (as explained in Dale M's answer):
Theft (Diebstahl in the Strafgesetzbuch, the German criminal code) is defined in §242 Strafgesetzbuch as:
Wer eine fremde bewegliche Sache einem anderen in der Absicht wegnimmt, die Sache sich oder einem Dritten rechtswidrig zuzueignen, [...]
Whoever takes movable property belonging to another away from another with the intention of unlawfully appropriating it for themselves or a third party [...]
So, just as under Common Law, the crucial question is whether you intended to return the money all along. If yes, then the definition of theft is not fulfilled.
However, if you originally did not intend to return the money, but did so anyway after you found it, it would still be theft. That would be for a judge to decide. A judge would probably look a the circumstances, such as whether you returned the money after you were found out, or before.
Yes, you purchased something without her permission or authorization.
Whether 1 dollar or 1000, if someone did that to you, right away you're going to be weary the next time.
Justifying an unauthorized purchase, no matter how small, will lead you to keep doing these kinds of things.
It's probably better to mention it beforehand, most of the time, people are willing to help.