Mary's birth pangs

 If Mary did not sin, how do you explain this Psalm 51:4-5

4 Against thee, thee only, have I sinned, and done this evil in thy sight: that thou mightest be justified when thou speakest, and be clear when thou judgest.

David acknowledges that he sinned against God.

5 Behold, I was shapen in iniquity; and in sin did my mother conceive me.

He also acknowledges that he was born in the original sin which we all inherit from our parents.

Is that what you were asking?

Hm? I think I get it. Are you saying that the Messiah is admitting that His mother sinned? Well, if that is the case, then He also admitted that He, Himself sinned. Read verses 2 and 3:
2 Wash me throughly from mine iniquity, and cleanse me from my sin.

3 For I acknowledge my transgressions: and my sin is ever before me.

Yes, this is a Messianic Psalm, spiritually speaking. But, literally speaking, David was talking about himself. Therefore, we must discern which portions of the Psalm pertain to Jesus Christ, which Scripture says:

Hebrews 4:15
King James Version (KJV)
15 For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin.

and Luke 2:21-22?

21 And when eight days were accomplished for the circumcising of the child, his name was called Jesus, which was so named of the angel before he was conceived in the womb.

22 And when the days of her purification according to the law of Moses were accomplished, they brought him to Jerusalem, to present him to the Lord;

Did you ever read in Scripture, where Jesus said:
Matthew 3:15
King James Version (KJV)
15 And Jesus answering said unto him, Suffer it to be so now: for thus it becometh us to fulfil all righteousness. Then he suffered him.

Although He never sinned, He submitted to Baptism for the remission of sins. To fulfill all righteousness.

That is the same reason why, the sinless Virgin Mary, submitted to the purification ritual, to fulfill all righteousness.

(ii) The Catholic Church teaches that Mary was conceived without the stain of original sin, then the woman with the 12 stars on her head in Revelation 12:1 is not Mary because of Revelation 12:2 the woman was said to be experiencing “birth pains”

The entire Book of Reveleation is written in a style of speech called Apocalyptic. It is recognized that this book contains many symbols. In this case, the pain of childbirth represents the pains which Mary did endure which are referred to in another symbol by the prophet St. Simeon in the Temple:
Luke 2:35
King James Version (KJV)
35 (Yea, a sword shall pierce through thy own soul also,) that the thoughts of many hearts may be revealed.

And this pain is not that of the physical birth of Jesus Christ. But of the spiritual birth of the Church by the efforts of Her Son, who also commanded her to be the mother of all who love Him:

John 19:26-27
26 When Jesus therefore saw his mother, and the disciple standing by, whom he loved, he saith unto his mother, Woman, behold thy son!

And commanded us to take her as our Mother:

27 Then saith he to the disciple, Behold thy mother! And from that hour that disciple took her unto his own home.

Which is also confirmed in Revelations 12:17:
Revelation 12:17
King James Version (KJV)
17 And the dragon was wroth with the woman, and went to make war with the remnant of her seed, which keep the commandments of God, and have the testimony of Jesus Christ.

The Woman of Rev 12 is definitely Mary.