You can be quite comfortable with your animals for ages, and then have an allergy show up at lightning speed. Cruelly, fur allergies always seem to show up in animal lovers. It could just be the fur, or something finer, like the dander.
If it turns out that you are allergic to your animals, have a word with your GP. He or she should be able to refer you to a specialist and there is a chance you could get a more effective treatment via a hospital - long lasting injections or similar.
With over-the-counter remedies, if the first one you try doesn't work, try others. A pharmacist can advise. Different anithistamine brands use different active ingredients and they don't all work for everyone. That can change over time too. I used to take Sudafed for my hay fever, but it makes me worse now and I've had to switch to Piriton this summer.