My chihuahua is trained to a puppy pad and stopped last month?

Take her in to the vet for a urinalysis to check for problems. While she’s there, get her spayed. There is no reason to have an intact dog.

Puppy pads are a bad, bad idea. You’re training your dog to have a preference for peeing on cotton. When the puppy pads aren’t available, guess what comes next?

Retrain the puppy to pee outside. And yes, get her spayed.

She needs a checkup at the vet, sometimes after going into heat they have a urinary tract infection or some nonspecific irritation. The first thing I always do with any sudden behavior change is go to the vet to rule out anything going on physically.
Also, pay special attention to what you use too clean up the accident spots, Feisty. If your little girl can still smell anything she will continue to use those spots. Locate a good enzymatic cleaner. (Nature’s Miracle, Get Serious)
I would suggest you spay her. Most vets don’t recommend the dog go through one heat cycle anymore.
Finally. About small toy breeds. Many, many people train them to puppy pads with no problems. It is certainly true in my case; Tita uses pads inside, grass outside. The only problem I have ever had is if the puppy pad is not changed sufficiently, the dog may go close to it. Don’t blame them, no one wants to step on that nasty pad.
Hope this helps; good luck!

From Cool Dog Training

Housetraining
Dogs create toilet preferences by surface. Grass and carpet are the two favorites because they are absorbent. Did the breeder have the puppy on grass? Consistency and repetition creates a desire for one surface – take your dog out routinely. If you do this right, your puppy will want to use only the outside for potty by the age of five months.

Puppy’s bladders don’t mature until they are five months old. At eight weeks, take your puppy outside every two to three hours day and night. Add one hour for every week of age. Restrict water intake at night, and don’t provide any water after 6:00 p.m. unless your dog is playing hard or it’s unusually hot. Take your puppy out for potty before bed. To isolate accidents, use a crate (which should be the happiest place on earth w/Kongs, chews, toys, and food). Never put your dog in crate as punishment or when angry! They will learn to hate the crate. Don’t open the crate door when dog is crying or pawing at door. Open when he is quiet. Teach your dog to go in for treats – throw one to the back, feed the second through closed door. If dog has an accident in the crate, make crate smaller (with just enough room to turn around) and think about taking them out more often. Accidents happen – clean with Nature’s Miracle or white vinegar. Do not reprimand your puppy – he doesn’t understand what you’re upset about! “Puppy Pads” teach the dog to pee on fabric (cotton & polyester) – don’t use them! Give the command, “go potty” or any other preferred phrase and stand still. When they squat and have finished, give them a party and a treat – yeah! If the dog hates the crate, try working with clicker training.

For the tough cases or for older dogs that have not been housetrained. They probably have not created a preference yet due to inconsistency. Restrict access to carpet, rugs, tile. Set up an exercise pen in your family room or somewhere close by. Line the floor with a heavy plastic drop cloth like painter’s use. Buy a strip of sod and cut it to fit into a baking sheet or cat litter box. Put small pieces of your dogs poop on each corner of the sod to give your dog the idea that it is a toilet. Hose off and rotate sod pieces daily.

Tie a jingle-bell to the door knob, strung at nose level, and teach your dog how to ring it when they want to go out. When your dog rings the bell on his own, run to the door, open it say, “outside.”

I second PnT. Puppy pads are a marketing ploy. Do the work. Take her outside every couple of hours!

we setup firehydrants (1 gallon water jugs) on these pads when we are away for longer periods of time but normally take him out every few hours or when he asks.