Despite some of the negative responses here, the "milk has been spilled" so now it's time to move on. You now need advice and not criticism!
It would be a very wise move on your part to take your dog to the vets, even if it's to just confirm that she is pregnant. The pet doctor, if they are a good one, will check her overall health and make any recommendations to you. If she is indeed pregnant then the following applies:
The typical gestation period for a beagle is 63 days, but she could have them anywhere from 60 to 65 days after becoming pregnant.
And I'm glad to see that you care enough about her to want to feed her right during her pregnancy. I hope you will carry that attitude long after the pups are born.
What she eats during her gestation period is absolutely important for her health, and the health and development of the "little ones".
To give the developing puppies the best start possible and to keep "mama" healthy, feed your dog a high-quality food, preferably dry, with some canned food every now and then.
Feed her a food which contains a "named" meat-source protein as the FIRST ingredient, with the meat being either in whole or meal form ("named" meats are listed on the ingredients label as chicken, lamb, beef, etc) Example: The ingredient label should read "chicken" or "chicken meal" and not "meat" or "animal" whatever.
The first ingredient should ALWAYS be a meat source. And the more meat sources there are within the first five ingredients listed, the better.
"UNNAMED" meats, by-products (whether named or not), and beef and bone meal are of poor quality, indigestable and generally unhealthy.
And definately avoid any product with, "animal" fat, "animal" digest, "meat and bone" meal, chemical preservatives (BHA, BHT, Ethoxyquin) and artificial colors and flavors.
Try to avoid products with corn, wheat, and soy as well.
Unfortunately many "top selling" dog foods today (Iams, Science Diet, Eukanuba, Nutro, Pedigree, etc) are not as good as the manufacturers claim, with some brands (Purina Dog Chow or Kibbles n Bits for example) containing many HARMFUL ingredients. That explains why they cost so much less than other better foods.
Fortunately there are still some good foods out there which cost just as much, and sometimes alittle less, as the more popular but less healthy brands.
Some good ones are:
Chicken Soup For The Dog Lover's Soul
Diamond NATURALS
Premium Edge
Canidae Canine
Healthwise
Natural Balance Ultra Premium
Eagle Pack (Holistic Select ONLY)
Innova
It's best not to buy dog food from stores like Walmart or Kroger. What they sell is cheap in both price AND quality.
It's also important to feed your dog proper amounts of food as well.
During the first 6 weeks of her pregnancy she generally will need very little increase in the amount of food you feed her.
But in weeks 7, 8, and 9 you may have to increase her food by around 25% to 30%.
Because of your dogs special needs, it might be a good idea to put her on a good quality puppy food during the last 3 or 4 weeks of pregnancy (she will need the added calories, protein, and fat that's in it). DON'T feed her any large breed puppy food, just the regular puppy or even small breed puppy formula's.
Just be sure that, whatever food you use, you don't overfeed her.
If you decide to switch to a different food, be sure to change gradually from the old food to the new over several days time (start out by using 3/4 of the old food with 1/4 of the new. Gradually decrease the old while increasing the new food over a period of about 7 to 10 days)
Check out the following links for any useful information. And good luck to you and your dog!