Could I have developed an allergy to the nickel fastening on my jeans?

Yes. An allergy to the nickel in the fastener is common, and there's no way to make this sensitivity go away; all you can do is eliminate contact with any nickel-containing metals. Choosing clothing and jewelry that have no nickel works best, but you can also paint the inside of the fastener with clear nail polish so the metal doesn't actually come in contact with your skin.