Yes, it is entirely possible that the Atenolol is responsible for your weight gain. I gained 50 pounds over the 10 years I was on it. Even diet and exercise didn't make a difference. My former PCP swore up and down that the med was not responsible for the weight gain. Long story short, I had an ablation several months ago for my heart arrythmia, no longer need to take Atenolol, and have lost a decent amount of weight since then. Perhaps there is a different med your doctor can put you on that won't cause the weight gain - you definitely should check with him.
I take atenolol, also and have not seen a difference in my weight, while on it. Have you had your thyroid levels tested? Under active thyroid can cause weight gain/inability to lose. Ask your doctor for TSH, Free T3 and Free T4.