Yes, atrial fibrillation can cause a BP fluctuation on the lower side. However, in your case there is a high to low fluctuation so other causes need to be looked into too.
Fluctuating BP is of two types: fluctuations on higher side and the other is fluctuations on the lower side. Fluctuations of BP on a higher side are mainly due to hormonal imbalance, kidney or heart problems. BP can also go to higher side after a physical activity like climbing stairs, running etc and this comes back to normal after rest. It can also be to due to anxiety and stress. Hence careful tests like kidney function test, stress test or treadmill test, ECHO, EKG, BP monitoring, and complete hormonal profile needs to be done. If the blood pressure is found to fluctuate during the day, then cortisol levels may also need to be checked.
Bp fluctuating on the lower side can be due to dehydration, certain medications, heart problems, untreated diabetes, nervous system disorders, hypothyroidism, hyperthyroidism, Addisons’ disease, hypoglycemia, VitB12 and folate deficiency.
Since I cannot examine you and know other related conditions you may be having, nor is a detailed history possible on net, I have listed the various possibilities that should be looked into. Please consult your cardiologist. Take care!