Okay, I work part time at a pet store, and I'm going to tell you something that might hurt your pocket book, but end up being really really great for your dog! Beneful is a horrible food, as is most pet food you find at the grocery store! All pet food is independently reviewed and rated and Beneful gets like two out of six stars or something. The reason your baby is straining to poop is because she is not eating a proper diet!
If you really are on a budget Costco's Kirkland Signature Premium food is a much better option, the lamb meal and rice formula is good. You always want the first ingredients to be meat, or meat meal. You never want wheat or corn in your pet's food EVER, those are the top allergens for dogs!
Also, I would steer clear of anything they sell in your vet's office (IAMS, Science Diet), they are better then Beneful or Purina, but vet's receive kick backs to carry them in their offices!
Other brands you could look into are Canidae, California Natural, Orijen, Natural Balance, Wellness, Innova...they are more expensive it's true, but a better food in the long run will save you money. They eat less because it's more filling, your pet produces less waste because they eat less "filler", and the ingredients are more nutrient rich which contributes to the overall health of your pet!
I personally have three dogs, I feed them a mix of Kirkland Signature Lamb Meal and Rice and rotate any of the above brands through with that. Research has also shown that mixing up your pets diet can be good for them too. You'd get tired of eating mac n' cheese all the time, right? I hope this helps, and if you have any specific questions please let me know and if I don't know I'll try and find out for you!
Also, I wanted to add that you should take her to a vet and have her checked just in case!