I think it is natural to have doubts at times. It may even be important. These doubts could come from Satan. He is your enemy... even when you are not a Christian. But if you are a believer, he really wants to attack you. In fact, he is regularly accusing God's people of untrue things. He manipulates reality to make us doubt our relationship with God. Do not buy the lie of the Devil. If you are in Christ, Christ is in you, and your fellowship is true.
But it is good to question your own heart from time to time. It is better to challenge your own motives than to slip into a fake walk with Jesus. Your heart is a tricky thing. The Bible says that our heart is deceitful above all else, and desperately wicked. We can fool ourselves into hypocrisy, or not being who we want or claim to be. So you should invite God to bring His holy searchlight into your life and reveal your true heart and motives. This is a way of keeping things right between you and God.
Perhaps today is a good day to pray this prayer that King David prayed when he wanted to be sure of his authentic and clear relationship with God, "Search me, God, and know my heart; test me and know my concerns. See if there is any offensive way in me; lead me in the everlasting way." (Psalms 139:23-24)