When I write this post, I feel sorry (personal opinion) for Sawda.
We can see that Muhammad married Sawda even before Aisha, after khadija's death. This is mentioned in Ibn Sa'd: The women of Madina vol-8 page-39 translated by Aisha Bewley
You can see that Muhammd had a desire to divorse Sawda because she became too old and Sawda knows she will not be able to fulfil the pleasures of Muhammad.
Muhammad divorsed Sawda and told her to start the waiting period in Ibn Sa'd: The women of Madina vol-8 page-40 translated by Aisha Bewley.
Sawda was afraid to loss her status, so she waited INFRONT OF AISHA'S DOOR (it was Aisha's day to have sexual pleasure with Muhammad) to see Muhammad to talk about this matter.
Sawda asked him is there anything about her, Muhammad dislikes; he stated "No".
So it is not Sawda's problem.
Sawda even gave her day to Aisha to please Muhammad. In that term only Muhammad take Sawda back.
Now read Sahih al-Bukhari 2593
Also read Sahih Muslim 1463a
So why did he wanted to divorse Sawda? Is there another reason other than she is old, (slow & heavy & tall which is mentioned in Sahih al-bukhari 146 and in Ibn Sa'd: The women of Madina vol-8 page-42 translated by Aisha Bewley)
Before Muhammad marrying Zayd's (Muhammad's ex-son) wife, Zainab. Allah has revealed ayah regarding the count of women a muslim could marry in Quran 4:3
And Muhammad was married to 4 wives ALIVE when he marry Zainab. They were Sawda bint Zam'a al-'Amiriyya; Aisha bint Abi Bakr as-Siddiq; Hafsa bint 'Umar ibn al-Khattab; Umm Salama, the daughter of Abu Umayya ibn al-Mughira.
Allah revealed ayah to allow marying Zainab in Quran 4:23 and after that the incident of divorcing Sawda can be seen in Tafsir Ibn Kathir - Surah 4. An-Nisaa . Ayah 128
You can see this alo in Sahih al-Bukhari 5206
So when you read Quran 33:50-52 you can see a Allah allowing Muhammad to marry unlimited amount of wives... [from more than 4 wives (in 33:50)]
Sawda died after Muhammad'd death in Ibn Sa'd: The women of Madina vol-8 page-43 translated by Aisha Bewley