Praise be to Allaah.
It is obligatory to cover the âawrah when praying, with something that does not show the colour of the body.
The âawrah of the woman whilst praying is all of her body except the face and hands. See the answer to question no. 1046.
If the dress is light and shows the colour of the skin, white or red, underneath it, then it is not regarded as covering and it is not acceptable to pray in it.
If it was light but did not show the colour of the skin, there is nothing wrong with praying in it. The same applies if it does not show what is under it except when you stand in the sun: if the prayer was not offered in a sunny place, then it does not matter, because the prayer was offered when you were covered.
This only indicates that the dress is light, not that it is see-through and shows the colour of what is under it.
Ibn Qudaamah (may Allah have mercy on him) said: What is required is wear something that conceals the colour of the skin. If it is light and shows the colour of the skin underneath it, and one can tell whether the skin is white or red, then it is not permissible to pray in it, because it does not cover properly. If it conceals the colour but shows the shape, it is permissible to pray in it, because this is something that cannot be avoided. End quote from al-Mughni, 1/337
Al-Nawawi said in al-Majmooâ (3/176): Our companions said: It is essential to cover with something that will prevent the colour of the skin from being seen. It is not sufficient to wear a thin garment through which it may be seen whether the skin is dark or light. End quote.
The Shaafaâis clearly stated that if (a garment) shows what is underneath it when there is sunshine or fire, and the âawrah can be seen in that case but not otherwise, then this does not matter. Similarly, it does not matter if what is underneath it can be seen when staring at it or looking at it from close up.
See: Haashiyat Tuhfat al-Muhtaaj, 2/112
Based on that, your past prayers were valid and you do not have to repeat them.
And Allah knows best.