Answer – I would say it is certainly possible. There is a very definite overlap between prayer and magic and a school of thought that they are actually just different ways of channelling energy. A group of people focusing on a single objective may well be able to produce results and people practising a faith (any faith) usually have a real advantage here because the belief in what they are doing is likely to be strong.
I would however imagine that if these people are proper Christians (by which I mean they accept the doctrine of loving their neighbour) that any energy pushed your way would be positive rather than negative. The fact that your neighbour said she would pray for your soul would suggest she has good (albeit misguided) intentions toward you. I’d say it is unlikely that this kind of energy would manifest itself in the negative effects you have been experiencing.
It is important not to overlook the psychological effects of someone threatening spellwork against you. Many witches use this to their advantage… If somebody believes they are being affected by a spell/prayer/working it is possible for them to experience the effects even if the spell has not actually been cast against them. So in your case the fact that you believe these Christians are working against you may have preyed on your mind and induced these worries which in turn have sapped you of your energy.
Probably the best thing to do is to be honest with your neighbour and ask her politely not to include you in any of her Christian rituals. Explain that you respect her faith but you consider her intervention to be an infringement of your privacy. I am well aware that in certain parts of the world even the most innocent forms of divination (angel cards ffs!) can be viewed with mistrust and suspicion so it is probably not worth your time explaining what you do and asking her to understand.
(I’d probably add that if she doesn’t stop you will do a bit of tit for tat and send something back her way but be aware this may not be an appropriate suggestion for your relationship with your neighbour or for the part of the world you live in. Sometimes it makes more sense to let it go than get started on a long running negative exchange.)
It’s definitely worth taking some basic protective action just in case this group are sending anything your way. Get some decent shields in place, salt your door entrances, immerse yourself completely in a salt bath and visualise any unwanted energies dissolving away from you into the water. In terms of spell work I would try setting up some reflective workings to send anything back at you to the sender. A very simple but effective way of doing this would be to make a small poppet of yourself and place it against a wall and behind a mirror. The strength of the spell will be the strength of your own visualisation in seeing the mirror projecting back anything directed at you.