As a part-time worker, you do not have the right to be paid for extra hours up to the normal full-time level. However, you do have the right not to be treated less favourably than a comparable full-time worker. Your employment contract cannot override that legal protection. For example, if your full-time colleague (whom I presume is doing a comparable job to you) is getting paid for all the overtime they do but you are not, that would almost always constitute less favourable treatment because of your part time status and would be unlawful.
Even if your full-time colleague is not doing a comparable role, the business still needs to be reasonable when they decide whether to pay for the overtime or not and to treat employees fairly. If they decide to exercise discretion in respect of one employee but not the other then this may be grounds for a constructive dismissal claim if you have been employed for more than two years. Although there is no contractual right to overtime, you would say that by treating you differently they have breached the implied term of trust and confidence. Also, if they have paid you every year except this one for working lots of overtime (even if it is dressed up as a bonus), they may be exercising their discretion unreasonably.
Similarly, it would be reasonable that they take into account factors which mean that you might struggle to do overtime. Depending on the circumstances, failing to take these into account could constitute discrimination if, for example, you cannot do overtime requested because you are caring for someone who is disabled or have childcare commitments.
I suggest you speak to your manager and discuss this with her. If your manager is unwilling or unable to change the overtime arrangements (including perhaps to increase your contractual hours for this short period of time), you could then raise a formal grievance. Ask the business to confirm in writing why you and your full-time colleague are being treated differently in respect of overtime payments. Keep a note of your discussions with the business as, if they are unwilling to make any changes to their practices, you may need to consider bringing an employment tribunal claim. You can bring a claim for the way you have been treated whilst still employed by the business. I recommend you take legal advice if you are considering this route as there are obligations to go through early conciliation and there are strict deadlines for bringing a claim.