I have a question that’s been bothering me for some time. I am 5 months pregnant Alhamdulilah and I haven’t prayed for maybe more than a month. I feel sinful and ashamed. I work out side and my body aches a lot. I feel extremely tired all the time and when I come home I just sleep. My back pain prevents me from standing for a long time. Is it permissible for me to pray sitting down?

According to traditions, a woman during pregnancy enjoys a great reward and her
rank is equal to that of a Martyr (Shaheed)
The Prophet (Peace be upon him) said:
“The reward of a woman, from the time of pregnancy until birth and breastfeeding is the same as the reward of one on the path of Allah, and if a woman leaves this world during that time because of the hardship and pains of birth, she has the reward of a martyr.” 
Keeping this in mind, you should be well aware that Allah expects you to be obedient to Him and do as much as possible to please Him and to earn His pleasure is but to your own benefit and perfection.
You have to see if you can withstand the pain without any repercussion, then stand and pray without prolonging it. If you think you cannot to that, you can do the qiyaam ( standing and reciting the hamd and sura) and rukoo while standing and sajda by sitting and keeping your head upon the turbah on a table or desk.
If you cannot stand and sit, then just sit and pray and do the sajda whilst sitting and keeping your head upon a table whilst doing the sajda. This is for you to decide how much you can bear the pain without any further repercussions like further pain.
Remember to at least start off to repay the qaza of the prayers left undone during the month you missed to pray. Feel blessed that Allah has guided you by reminding you of the importance of prayer through the guilt you have of having missed your prayers. Remind yourself that you have been missing the sweetest conversation and act in this world and be smart to get hold of it before its too late and shaitan gets a chance to overtake you. He doesn't want mankind to benefit from their lives nor does he want them to have the utmost pleasure. Prayers is one thing that if you make it with concentration, you will benefit and have deep satisfaction in your life as well as the life of your child to.be InshAllah.